what happened to the XR600? I'm 5'11, 180, and I rode a 91 or 92 (can't remember now, it was borrowed) XR600 for about a year. It was heavy, took a while to learn to start (but I eventually did, sort of), but I have to admit it was kinda fun to ride. As long as the trail didn't get too technical, you really couldn't feel the weight. It was very plush on the trail, had a great motor, and never once crapped out on us in any way, for as long as it was owned by my friend (about 4 years). We hardly touched it with a tool or washcloth. I think he adjusted the valves once. And it was street legal, too. That was really fun!. He sure could blaze on it!
For me, it wasn't the greatest bike because I was still learning, and when things got rough (he's a pretty good rider, and always lead me down the gnarliest trails) it proved a handfull. But, if I found one today for that price, I'd consider it- I had to sell my '00 CR250 last year and am trying to save $$$ for a new(er) bike to buy within the next year.
I'd guess any other bike, for that little money, is going to be a roach. Trust me, I've spent $1000 on a bike before. It cost me another $1000 to keep it going until the engine finally grenaded. Remeber the old saying- you get what you pay for. In this case, that XR is probably the only exception. Stick with '87(I think?) or newer XRs (for the better technology they come with) and you'll be fine. And I can't think of any place that a 4 stroke is allowed that a 2 stroke is not. Green sticker is the issue, not engine type.
Regarding the 84 XR350- I don't know anything about them, but my friend owned one, and said it was a nightmare on the trail. Or maybe it was an XL.
Good luck!