1. I had a Sioux gun that died. I bought an ACAT gun from The Yardstore and it's excellent. Feathering trigger, reliable, etc.
2. Don't know the RV-14. I use a 2X on the -3 rivets and a 3X on the -4 rivets. For only one gun, I'd get an ACAT 3X. For more on rivet guns take a look at this thread.
3. C-frame works quite well. No need for anything else.
4. I have one and it's my go-to. It has square faces, and one slanted face would be handier. But sometimes I use a small steel L bar because it fets better in that case, and less often one of the other steel bars.
5. It's not wide enough. It's too easy to accidentally mislocate the part and drive a rivet off the plate. I don't know what's needed for the RV-14, but once I borrowed an RV-10 buddy's 6' long plate for something. Once. And since I'm building an RV-3B, there are more unusual things going on than a well-thought-out RV-14.
6. One minimum. I have one for the #30 pilot, one for a #40 pilot, and one for a #19 pilot (for 8-32 screws) would be handy too. But the RV-14 might have different requirements.
7. I like a swivel set without the guard.
8. As long as he gets Cleaveland dimple dies, he's doing okay. I have the tank and substructure and use them. Are they necessary? I don't know.
9. Never used a Numatx. But my pneumatic squeezer works fine and no reason to spend more.
10. I have a 4" one with the hole and it's my every-day one. I have a small one, can't remember its size, that once in a while is necessary to use to fit something where the 4" is too big - caveat, the RV-3B, my project, is smaller than the RV-14. I've heard that a 4" no-hole yoke would be handy in a few places and it would have been, but I don't have one. The longeron yoke is essential too.