I am a noob to 2 strokes but heres my question

I am currently looking for a MC21. The ones i am finding either have alot of kilometers or almost too little to be suspicious. Anway what should i be looking for, meaning problems, damage when buying a NSR? And is maintenance expensive for this bike? What needs to be changed? weelky,monthly,yearly??Oh and also if the bike has around 25000 clicks should i be in the market for a new top end?

Thanks