92 with 166k

I checked out a '92 Si today with 166k on it with asking price of $2800. Test drive went well and nothing seemed wrong. Clean inside and out. No after market parts or anything on it. Only thing that I saw "wrong" with it is when the owner drove it away I saw some grey/white smoke coming from the exhaust. Now from reading other posts this could be bad spark plugs or oil leak in engine or head gasket or nothing, correct? How much does it cost to replace the head gasket, expensive or not? I'm thinking of offering 2k or even 1800 if there seems to be a problem then fixing it up so it will last another 34k miles. Seem reasonable or not worth the trouble?