
Without looking at the car and your definition of "mint", it is hard to say if an 86 Fiero is worth $5000. With a rebuilt engine that part of the mechanics should be good. Where was the car used? If the car was used in the north and in the winter, there may be space frame rust damage. One easy way to check is look at the underside or pull back the sides of the trunk carpet to look at the frame rails behind the carpet. Have the brakes been replaced? Has the suspension been replaced?
An AC compressor replacement can get pricey. The cost of the AC system for my 87 GT was more than $1000. Using rebuilt compressors have proven to be an expensive mistake. The rebuilt ones have only seemed to last a year or so. (did that three times on my 88GT)
All my Fieros have more than 100,000 miles on the odometer. I have a friend who has an 86 SE with more than 260,000 miles on the odometer with the original engine. (A well maintained car)
I would suggest that a 21 year old car be used as a "fun" car and not as a daily driver. At least have a backup vehicle around in case something goes wrong with the Fiero. Unless you do all the maintenance and repairs yourself, find a mechanic who is willing to work on a Fiero.