Well the biggest challegne would be to find a capsule that is spot on to get c800 sound. Yes that Manley capsule is very close, but i doubt you could get closer without buying the actual sony capsule, which costs an arm and a leg. The capsule is the most important part, then head...
As an aside to this thread and to resurrect it slightly, I'm surprised that there hasn't been a DIY C800G clone I've seen - please let me know if there has - given that the same capsule was used on the C48. Given the going price for C800Gs, I'd have thought a project simila...