I'm looking at buying An 1984 cheetah 166. It needs a new transom seal/boot. How hard is it to do/ I've never done one this old. IT's a older omc sterndrive
First recommendation would be to look for another boat. Old OMC drives are hard to get parts for and even harder to find knowledgeable mechanics, let alone one who'll even work on one.
Look for a Merc or Volvo boat and your boating days will be less stressful.