there is one other difference that hasn't been mentioned, yet. on the v'6 cars, there is a change in the routing of a coolant hose to the water pump. in '86, it was routed in a channel that runs along the tire well and under the battery and comes out beside teh altenator and attach's to the top of the water pump. in '87 and '88's it connects from the back to the side of the pump.
as for buying a car... you can buy a nice '87 or '86 GT for around $3500, and upgrade the suspension for about $1200. also, bearings are easier to find (really the only thing that would probably keep me from ever buying an '88)
i have an '86.5 with a rebuilt suspension, owned teh car for 13 years, and i can telll you she handle's like a dream!
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hoop
Red
'86 GT
5 speed
Borla Exhaust
Eibach's/Poly/Kyb's