When I started this quest ... that's where I landed (a monolever R80RT). However, If I had to do it again, I would find a dual shock R80 or R65. Why you ask?
1. The dual shocks have THE WORLD of final drive choices. With the mono, you have three.
2. A R65 can be had CHEAP. I've ridden an R65 which was very close to the output of an R80. Plenty of fun in the woods, and if you keep the RPM's up it was enough on the highway.
3. You can still do the big forks up front, and extending the swingarm has the same issues as a mono.
4. Don't plan on 'selling the RT faring' for a quick $300. Nobody wants them anymore.
You will not find a G/S cheap. That's the fact, and if you did it would cost a large sum to make right. The RS's are typically more money than the RT's. The GS's are getting more rare, and you have to lengthen a monolever swingarm on those just so it fits correctly. Stephen Botcher did it ... and you need to read his page. http://www.stephenbottcher.net/index.html
Here's his diatribe on extending a swingarm (and parelever->monolever conversion): http://www.stephenbottcher.net/BMW/paraconv.htm
At the end of the day I would recommend several things:
1. Don't mess with the geometry of the frame. Let a pro do that. The frames geometry is perfect for the weekend warrior. If you want a race bike, buy a KTM.
2. Buy 'complete' parts. I was able to buy the complete front end of a DRZ400. I didn't have to cobble together brakes rotors, master cyclinders, wheels, axles, mounts ... etc.
3. Make it work first. Don't get all neurotic about the 'perfect seat' ... find a board and tape a towel to it, and go on a ride. The longer you spend 'off' the bike' ... and not riding it ... increases the chances you won't finish it.
4. Do ONE project at a time. Do the forks. Finish the forks. Ride it for a few days. Work out the carbs. Finish the carbs ... Ride it for a few days ... etc.