Go here:
http://www.pelicanparts.com/911/technical_specs/911_guthrie_faq.htm
to learn more about each year of 911. Linked there is a checklist to look at before buying one. Make sure a mechanic who is an expert on old Porsches does a thorough inspection before buying.
Sounds complicated, but the end result will be worth it and save you lots of $$$.