Well an 01 RM125 is an awesome MX bike. Motor wise what you want to do is replace the top end immediately. Thats the 1st rule of buying a 2T. 2nd rule is don't believe anything the seller tells you.
Things you should look for are misplaced nuts and bolts, non factory bolts (hardware store bolts) worn engine cases, stripped bolts etc etc. Anything Cosmeticly faulty.
I always look for spooge around the exhaust. If there are significant amounts of spooge around the silencer and around the exhaust port then chances are he just rode the bike and not to its full potential which is good. It means the internals are not that beat up.
A good used RM125 in this economy will hit you at about 1100..
My buddy Tony has an 01 RM125 and that bike is FAST!!!!!! ride it hard, maintain it and that bike will shoot you to the moon!