The money sounds right. Still want to check all the obvious things if you can, compression etc. I think that's a twin shock model. Either way, yeah it's a cool bike, but it's not an XR100, smaller wheels, 16" front 14" rear I believe, VS 19" and 16" on the XR100. Probably about 6 horses instead of 8 for the 100. You can still get it around Aaron's track, just don't try to do what he does. It'll end in tears.
If it's an unmolested bike it's definitly worth it.
evil, itd make a great pitbike for you, but dont even think about running that thing around arrons place. they are waay smaler than the 100's but actually alot of parts interchange. that one will have a points iggy too. not so great. 300 is a good price. but they arent really built for abusing them the way you will. although i have a feeling bransons got a few sweet speed parts for em.
I think you could punch it up to a 100 pretty easy. I think powroll has some hop up parts for them. I have an XR75 engine and frame here. Always wanted to build a 50 class bike (81cc legal limit) with it. Five speed too.