bike for me? rm125 ?!

pretty old bike, but a new top end is allways good-- since you wont be riding the crap off of it, I'd go for it.

Sprockets are typically $35 rear, $15 front. I would recomend an O-ring chain which would run about $55-70 (all USD$). When you look at the bike check for play in the chassie bearings as they can add up if they all need to be replaced. Try to rock the wheels back and forth (perpendicula to the bike). If there is any movement the wheel bearings are shot (~$50 per wheel). Put the bike on a center stand and see if the rear wheel/swingarm has any up and down free play (moves without compressing the shock). If it has much free play then at least on bearing in the linkage/swingarm/shock needs to be replaced. Laslty, while the bike is on the stand check that the handle bars move freely from lock to lock then check that the front end has no play at the steering head. Roughness or free play means it probably needs new steering head bearings. If all these things check out good, you have found a very solid well maintained bike:)