Well, let's see...
1. Standalone with caveat.
I have the Stingy Gamer Edition of the Revised BESMd20 rules. There's been a couple of instances where I went and picked up the SGE to figure out something in Everstone. If you've got the Anime SRD you should be covered. One of the bits for example being War Golems. You're given fixed stats for 'em. The problem is that for the kind of city-based game I'm going to run, 12 foot tall, 1600 LB War Golems don't quite work. But it's not explicitly said how many points the golems were built on.
Waaaay back at the beginning of the archives of the yahoo group, Jason Moon says:
http://games.groups.yahoo.com/group/Everstone/message/14
I actually used 6 character points to build each race. Except the Gavown which only uses 5 character points. Thus when building a character with the BESM d20 book you would have 46 points to spend which would leave you with an extra point for Gavown characters(which should most likely be spent on a Gavown attribute).
2. I _think_ it's built from the 3.0 version. There's a background feat called Ambidexterity, which as I recall was dropped from 3.5. I'm not really familiar with 3.0 though.
3. I think you're solidly covered.
4. Hmmm... sorta tricky.
The Lannith companion can still be bought on RPGNow. You may or may not have a use for it. Mainly it's some adventures, and a mundane fighter template which is almost exactly like the Warrior template from the Everstone Companion. You get a map of Cinder, but it not like that's really a big deal.
As far as specific BESMd20 products are concerned...
The Monstrous Manual _might_ be of use to you. It's got a whole bunch of d20 critters that have been converted over to use the BESMd20 system. You can find copies of it for sale online pretty easily, although not usually discounted.
Having said that, you can buy it from Titan Games for $15 including shipping. Just search ebay for "goo02-605" (minus the quotes) and you'll get the link
As a sidenote, the author of the book has an.... interesting... post here:
http://forum.rpg.net/showthread.php?t=277788
It's up to whomever as to how they want to spend their money, I'm just pointing out both things for folks to think about before they spend their money.
You could go with the Advanced d20 Magic if you wanted, but I personally don't see a whole lot of benefit to doing so. It depends on how much you're into the traditional style of magic (what BESMd20 refers to as Dynamic Sorcery), and if you're looking for the more traditional classes or not.
If you're planning on playing/running the Everstone world (or one similar to it), then neither book is super important. You'll probably get more milage out of the Monstrous Manual.
If you're planning on doing more traditional fantasy gaming, but using the Everstone implementation of the BESMd20 system, then I can certainly see getting both books.
If you plan on doing mecha stuff, or you _really_ want to go all out on doing golems... the Mecha SRD is a pretty good way to go. If you're looking to spend money, I'd get the Dream Pod9 Mecha Compendium Deluxe. It's hardcore mecha building complete with settings and a pretty detailed breakdown of components and so forth. GoO let them use the Mecha SRD to power the DP9 mecha book, and DP9 actually managed to publish theirs before GoO managed to publish. DP9 then went back and added a detailed breakdown and some more settings, and released the revised product as the Deluxe version.
And they've still got the summer sale going, meaning you can pick it up for $20 from them. Just make sure you buy the Deluxe version.
http://www.dp9.com/Products/D20Mecha.htm
I'd probably follow the suggestion in the book and make golems based on around 200-300 mecha points. You'll have to decide on what exactly feels right for you.
Another advantage to using the DP9 book, is that you could reverse engineer the landships/rune vehicles a bit. Rebuild them using the mecha point system, and then use that as a guideline for building others.
I mention this because it appears as though the Shone (the first to stand up to the Hollow Lords) have some sort of magitech equivalent to power armor. So if that's something you're going to keep in the game (or play around with or whatever), it'll give you a consistent method of keeping things on the same relative playing field.
If you don't want to go all out nutty like that....
Factory and Armory, both from Perpetrated Press. Really groovy books. They're written from the point of bringing magic and technology together. I originally picked 'em up to use as additional stuff for Dragonstar, but with a tiny bit of tweaking, you should be able to get a fair amount of use out of them for Everstone. You can find them for sale really cheap on ebay, and for some reason they seem to be tossed into lots. For example, searching ebay for "d20 lot" came up with this one that ends in a day and a half:
http://cgi.ebay.com/Mixed-LOT-of-4-...ryZ44112QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Arsenal will get you a variety of additional weapon stuff that can be added, and Factory would be an alternative way to handling golems. While I like 'em both quite a bit you don't really _need_ either of 'em. If you're looking for more weapon/armor stuff though, Arsenal is a pretty good way to go.
If you've got Dragonstar stuff, you could probably pillage some of it as well.
Alot of it really depends on how much magitech you want in your game. Myself, I don't plan on adding in anything really, at least not for a while. If there's a strong push for some additional stuff, then I might do little bits here and there but not too much. A different version of a Land Runner here, a couple of more weapons there.
I do have Chaostech thanks to Ptolus, so I think I'm going to use it as the basis for some of the Vor stuff, but especially for the Adone stuff. There's not much information given about the Adone, other than they keep trying to infiltrate the Magi Guilds, and that they seem to have tech/equipment similiar to the Rune stuff, but it's demon based. Sounds like a perfect match with Chaostech to me.
Like I said, it's kinda tricky. It depends on where you want to push Everstone as to what's going to go best.
5. Hmmmmmmmmm.
Well, obviously there's some incompatability because Everstone has some different assumptions. Armor providing DR, players getting a defense roll, spells over 5th level are inappropriate, and that sort of thing. Everstone does keep the BaB approach (unlike BESMd20 which does combat as a skill) so that helps a bit. Just from what I've been looking at I don't see any really _glaring_ problems. I guess it depends on what you're looking at.
One thing that I'm a bit iffy on is the whole CR system and Everstone. I want to try looking at something like the Sun Magi, and see what one roughly works out at in terms of it's CR level. I'll be using Upper_Krust's CR system which can be found here at Enworld, as well as in the Grim Tales book (which I have).
My impulse is to say that an Everstone character is stronger than an equivalent D&D level character, I'm just not certain how much stronger.
Everstone does explicitly allow for the traditional style of D&D magic item. In fact, they're actually even more valuable, because all the magic stuff in Everstone is powered by Energy Points. Whereas the usual D&D magic stuff isn't. Which is part of why you've got Templars going out and questing for their magic armor and swords for example.
Bringing other classes into Everstone....
That could be tricky. I've been thinking about it myself. Everstone claims any class can be used, it's just the usual D&D/d20 class is more restrictive.Of course, that's because the classes are built on very different assumptions.
If you're simply looking at it purely from the mechanics perspective (the Everstone implementation of BESMd20) then it shouldn't be too hard. Especially since you've got a number of the base classes already worked out for you.
If you're looking at it in terms of something like a Dragon Shaman from the PHB2 being imported into Everstone and how it's going to work with Everstone classes like the Magi, that's where I think it might get a bit tricker. Still workable, but trickier.
6. The book seems pretty well put together. Hardback. Interior pages aren't glossy and it's all black & white.