CHASSIS,CHAMBER SEALED ENGINE (PREFERRED),TRANSMISSION are the first priority...Personally I would suggest you to go for a brand new 125 ...somfor checking the engine go with a trusted mechanic,listen to the engine's sound carefully rev it up a little ignore the if the noise is of tappet and spark plug its ok.Go and test the bike slowly in a silent street so that you can check all the noises.check the gear if they are making noise or slipping,in short transmission should be perfect.Check the chassis thoroughly and also check for accidents welding.DO NOT GO FOR GOOD LOOKS for eg.chrome,fuel tanks and things like that.TO CHECK THE CPLC GO TO THE NEAREST POLICE STATION OR ONLINE VEHICLE VARIFICATION WEBSITE.