That is true to. On another note I ran Ravenloft every weekend for a year without the DMG. So the DMG is by no means necessary, but I still liked it and am glad I bought it. The DMG is the only 3.5 book I own, and I haven't found anything difficult to "upgrade" to 3.5. Of course I haven't found much that needed to be upgraded, except for monsters and some spells.
So I would also go the cheap way if I was you. As for the Gazetteers, those books are nothing exceptional. They provide you with setting information is a semi-entertaining format. There is nothing in them that is in anyway essential, unless you feel it is important to run campaign settings that are as "canon" as possible. the equipment book was useful to me. the dark, light, walking undead, and fey books really didn't add a whole lot to my campaign. There were things I used as DM, but my players largely ignored the material. So I wouldn't spend money on them unless you really want more inspiration for your Ravenloft campaign. They did give me a lot of that.
As for the monsters in Denizens of Darkness, I didn't find that to be the case, but I do a lot of "editing" on the fly, so any problems i had would not have been written down by me. So, as a result, I wouldn't/don't have a catalogue of problems for the books. Also a number of the monsters are very powerful, even for their CR rating; but I found that fit well with the basic concept of Ravenloft, which is the monsters are supposed to win. I also found a fair number of the monsters were pretty underpowered for their CR as well. All were easily edited in my opinion.