well the cost of parts. obviously. it depends where you buy things from.. i would be careful with ebay of course.. unless the parts are actually made with a thought that they will be put into a bmw.
exhaust can get pricey.. again it all depends.. you can spend a few grand from AC or like 600 bucks from BavAuto. Also, one thing, KNOW that the cooling system in the M20 is a BITCH to bleed. Maybe Im retarted but it took me 1500 tries to get it just right. kinda. once you open up a hose somewhere or anywhere in the system, you gotta bleed the air out..
Do you plan to do any suspension work to it?
I think that for that price, its a pretty good deal.. I mean as long as the engine is still good.. you may want to do a leakdown test or a pressure test on it too.. should be alright though. M20's are said to be bulletproof