I'll go along with Bigifig on this. A modified collector's car is worth less on the market than an original. Sometime the mods are minor, such as different wheels, radio, pulllies. These can replaced if the owner kept the original.
When you get into mods, such as headers. rebuilt heads, trans, suspension. The value is gone.
With that said, the 89 turbo are a hot collectors item because of it's built on the last of the Buick turbo V-6s. Were and always will have high value if kept original.
Now, if your looking to invest. Your better off buying stocks or bonds. For example, the 1979 Trans Am 10 year anni. 4 speed sold for about $ 12K if you could find on then. Today, on E-Bay, a perfect low mileage example will go for around $ 25K to $ 28. If you less out the storage costs to keep something that big for 25 years, Good investment? Up to you.
If your looking to have a fun car to drive, look at the late (98-02) F-bodies. A buddy of mine just picked up a Z-28 6 speed in red with 35K miles that looks show room new for $15K.
Its up to you. One last tip. Something people think what they have is worth more than what it is. Its up to you.
Take care.