thanks very much for the advice, repair shop estimates for engine rebuild range from 800 to 3.5K depending on what they find inside. not including trans.
these bikes in pretty nice condition sell for about 2.5K, so i reckon i'll take the bike if the current owner pays me $500.
1976 r75/6 -- mine since 1980, thx lloyd!
1985 vf500f -- v4, 6 spd, 16v, triple discs, water cooled
1997 900ss -- full sp spec