£120 is alright for that, but if you really tried, you might be able to do a bit better. It it's in a really good condition, then i'd be tempted to get it. If stuff needs adjusting, or is wearing out, then try and haggle him down. Providing the drivechain, and brakes are good, the worst part of the bike which i can see is the fork. However, a pretty nice second hand xc fork, will only set you back about another £50 or £60, which you probably won't need to spend anyway.
Cheers