Kx 125 or xr600r?

Hi guys i am looking at a few bikes locally and I am down to two of them. The first is a completely rebuilt 2001 KX125, it was stripped down and just about anything with wear was changed. It looks brand new and only has 2 tanks on the new engine and parts. It has brand new tires also. It looks about as new as a used bike can look. He is asking $1400 for it. Second choice is a 1986 Honda xr600r Baja Edition with a completely rebuilt engine he is asking $850.
I am 6'4 250lbs and have little experience. I plan on riding carefully on open trails for awhile while i learn more. I would like power but i am honestly not even bored with riding a klx 125 as of now, just uncomfortable. The xr would most likely be easier for me to learn on and get better gas mileage and have more torque and last longer but it is air cooled and i would be afraid of it over heating on me during summer monthes. I am attracted to the kx because i could rebuild it myself for cheap and learn that aspect of the sport. Plus, its like a brand new bike and would have good resale value. I think it would be fun to rip on a 125 2 stroke also but i am worried it wont have enough low end grunt to pull me around and i would have to 'give it hell.'

I know i am comparing apples to oranges but i honeslty think i would be happy with either one. For the record i have not ridden either or any bike other than a klx125 for that matter so im sure either will feel like a whole lot of bike to me. Any input would be nice, i cannot make up my mind and both seem like decent deals to me. thanks in advance.

Are parts readily available for that old of a xr600r? I am almost leaning toward it because im sure i could get a year or maybe even two of trouble free riding as long as i maintain it which isnt bad for a $800 bike. Anybody have something similar that can tell me if air cooled, large motors overheat easily or would i have to really rip on it?