Old fart chiming in.
I presently have an '88 GT T-top, and find it to be the best year, but.......
Find one completed, and in very good condition to begin with, you will be money ahead for sure.
Mine I purchased as a project, and knew what was ahead of me, so take the rest for what it is worth.
I have had '84's, '85's, '86's. and '87's all brand new back in the day.
All of them were really fun great cars, but there is something special about an '88.
My first '88 was a Formula around 12 years ago, I loved the difference in handling and braking so much, I bought another Formula soon after selling that one.
There is a difference from the earlier years.
When it comes to parts, '88's parts are really not that hard to find, but if you find one that needs a lot of things, stay away.
Good '88's are not that expensive at the moment, and that is the key I think, find a really good one and maintain it.
Now, T-tops.... hmmmm, parts can get pricey! http://www.fieroparts.com/
Tail lights for a GT, pshhh, don't even ask, lol, otherwise there are a lot of common parts with the earlier ones that are available, or being reproduced to a rather high standard.
I highly recommend looking here http://www.rodneydickman.com/catalog/index.php
and here http://www.fierostore.com/
to get a better idea of what is out there before buying one.