I'm going to look at a cheap but good conditioned gt fiero. It has a new motor with almost no miles on it. Was told it need the ecm or a new wiring harness. How hard would it be to get that or to replace? Would it still be worth it for an 87 gt?

for a 87? Sure it would. It would be worth it. You can get the ecm off of fierostore.com

definately. an ecm from rockauto is under 50$, if it is even needed.....