There are a number of CMC owners on here, including me. The main issue with any CMC is always "Who built it?" There were a few (very few) that were built (supposedly) by CMC, but the reality is that CMC contracted with a few small shops in the Miami or upper Mid-West to do the assembly for them. Generally, the quality of the fiberglass is quite good. Not terrific, but quite good and it generally tends to be thicker, rather than thinner. The quality of the fitment of doors, hood, engine cover and everything else has everything to do with who assembled it and how much care they put into their work.
Depending on how it was put together, the body may develop stress cracks surrounding the headlights (generally eliminated if the front body mount was properly installed) and a few have exhibited a strange bubble (more like a shallow dome) between the rear seat and the engine cover. The body has an integral steel frame to provide strength to the fiberglass, and it also reinforces the shortened VW pan, making it quite strong. When it is assembled properly, the car doesn't creak or groan or snap or rattle when driven. Overall performance has everything to do with the power train, so any engine from a 1,776 to a 2.1 liter will do just fine, although larger engines give better performance.
Perhaps the best approach would be for you to find someone on here who has a bit of experience with Speedsters, and CMC's in particular, and who lives in the area of the car you're looking at and ask them to take a look at it with you. That way you'll get some experienced eyes who can ask the important questions for you.
The Speedstah Guy from South Carolina