If you have FF compatible lenses already, I'd suggest getting the FF camera first, and then deciding on a fast prime later. FF + fast primes give you thin DOFs, which you can use creatively to good effect. How you use each lens will chance when you move from APS-C to FF, so get the camera first, then use it and then get the lens at the focal length you use most.
It's true that the 50L is not the sharpest, so try it before you buy. It has other qualities, but it is up to you if it is worth it. I don't find it as sharp as the 35L or 85L, but I find that it AFs much better than the 50 f/1.4 especially at larger apertures, where it is more critical.