Can you build a whole car from aftermarket repop parts?

Not with an A-body. But with an E-body, it's getting pretty close. AMD sells everything on the outside, fenders, quarters, roof skin, front and rear frame rails, crossmembers, upper and lower cowl, rockers, all of it and then some. The inner roof structure, inner quarter structure, and inner door frames (they sell the skins) would be all that you needed. And although I haven't seen these in person, if they are what they're advertised as you could do it. You'd still need to find some stuff, trim pieces that aren't made, gauge panels etc, but you could create the entire shell and then some.
New DII Inner Quarter/Door Frame Assembly - LH Left Driver, D-6099WT