I could go on and on with this topic being one that enjoys playing around with fiberglass, carbon, kevlar, and making molds. Going to SEMA and being able to closely inspect the GT-40s there did not impress me. I'm too anal when it comes to these cars. Therein lies my personal dilemma as the real, vintage cars were never near perfect, so I have to settle for leaving my work at about an 80-90% of my standards and walk away from it for awhile as it kills me sometimes. I also understand the cascading effects of how subtle changes in one panel can affect the fit elsewhere, panel buffeting/vibrations at speed, stress areas, and so on. Some forward thinking is needed/helpful. Door gaps and seams, however, were a non-negotiable item and I try to get these well enough that when you see a reflection across the car, across the roof line, the image does not get distorted or offset from seam to seam. It also provides me with a little consolation as I walk away and look back at the car, it gives me pleasure. ;-)
Understand that fiberglass just moves/migrates with time, heat, and cold cycles.
The $15-25k is a realistic number from what I've seen, and my experience doing all my body work and paint, but don't expect the end result to be perfect as these shops can't afford to work on your car for a year or more, letting things properly cure and settle from step to step. I've seen at least one of these "$25k" repaints and still find flaws with it but the owner is happy with it and that's all that matters. Just understand that no one, except maybe the owner/builder (you), will put the blood, sweat, and tears into building/finishing a car like this.
I would highly advise you get your suspension/wheelbase set and finalized, doors, hinge work, bonnets, to match to the best of your abilities before releasing it to a body shop. That is your foundation that the body sanders, panel techs, and painter will use to start from. Then be ready to miss your car for awhile. Consider disabling the car so it can't go on a joy ride (happened a twice to me with other cars). Better yet, pull the engine/trans back out and do the finishing touches from home to keep you busy in the meantime!
Good Luck!