I can only give you some input.
Both kits are constantly being cracked down on by Ferrari reps. Unlike the multitude of Lambo kits out there, you're likely not going to find an exact wheelbase replica of a F40 or F50. I don't think an exact F50 replica has been built, and there once upon a time was exact F40 kits being built, but not many of them got out. Most of the F40 kits use stock Fiero wheelbase.
One of the best builds I've seen is ferrobi's excellent F40 replica -
https://www.fiero.nl/forum/Forum3/HTML/000034.html He can probably give you many tips when you finally find a car. His car is becoming very exact as well. The F40's interior is (relatively) simple, compared to other replica car's dashes and interiors.
I can imagine the biggest hurdle you'd come across would be if you bought an F40 kit and had to find the rear hatch window. The ones that are produced I believe are made out of lexan, and the real thing has the vents in the window cut open.
An F40 will be somewhat easy to find replica styled wheels for, but there isn't an near exact F50 replica wheel set built, so you'll have to use a bit of creativity there.
