Lots of good advice in this thread.
I'd write as much as you can and as well as you can and leave it at that. Maybe your rough draft will be better than someone else's polished draft. Maybe not. Telling non-writers how much they have to write has never really worked, from what I've seen. If you're a writer, write. Get it done. You know what you have to do. If you're not a writer, no level of logical reasoning is going to turn you into one.
I have two novels that are already sitting in publisher slush piles, and have been doing so for a year apiece. I'm likely going to withdraw one of those, particularly since the publishers have a lousy track with losing my stuff, and submit one to Tor. Either that, or I'll submit the novel I'm currently in final-rewrite stage on -- it's almost ready to go to publishers, and it will certainly be in shape to do so by March.
Not counting non-fiction or gaming articles (or grant letters or technical writing or letters I write to my mom, or my blog, for that matter), I've got about 10 sales -- two of which still haven't gone through to publication but will hopefully do so early next year. So I'm published enough to know what I'm doing, but I'm not a name.
There were other threads on this topic, and what the rights-thing roughly boils down to is this: It's no longer your world. If you want to do sequels, better hope WotC likes you. If you want to sell foreign rights to your book (which makes you a lot of money in the long run), tough luck. If your book gets turned into a movie, a video game, or a Saturday morning cartoon show, you see no money.
A lot of writers on writers boards are discussing this and coming away unhappy and saying that "no professional writer would go for this". I don't agree with them -- pretty much for the "It'll be on the shelves in Borders" reasons you mentioned, Two -- but I think it is a valid reason to be wary. I write my books standalone, or at least "can-stand-alone", so I'm not giving away a world I had a 10-book series planned for. Nevertheless, I don't intend to give them a world that I have specific plans for in any way, if that makes sense.
As for the snarky comment about "Oh, it didn't sell at Tor, but it might sell here" -- try submitting. If you don't immediately get rejected by the unpaid intern reading the slush pile, you're looking at a return time of more than a year. Wizards might have a shorter turnaround time, and while I won't send them something I have a ten-book series planned for (or, in other words, I won't send them my "this is what I want to write for the next decade" series), I will definitely send something good but floundering in the slush at Tor, Baen, and Daw over to Wizards for a look.
Word lengths:
I've heard that Romances are in the 70-80k range, Mysteries are in the 80-90k range, and Fantasy is usally in the 100k range, unless you mean Big Fat Fantasy, which starts at huge and gets bigger from there.
If we're talking SF or Fantasy, 80k is not going to get the door slammed in your face as too short, and 130k might be the highest you want to go before your novel's length really becomes an issue. Those are utter guesses, though.
Good luck to everyone!