1. If you are refering to the form factor, it should be ATX, micro-ATX, mini-itx or what you want, but it should be standard.
2. Why not both?
3. No. They are slow, expensive and outdated.
4. No. I would prefer an Amiga usb (or ps2) mouse and keyboard. You could do them with exactly the same shape, colour,etc of the originals. That legacy ports would increase the cost with little benefits. Using only usb would make it easier for newcomers.
5. Probably usb. Although I have not anything aginst ps2.
6. 92? I guess you mean 96. It should be exactly like the old keyboards but with usb connection.
7. Go for the most modern chip. Design time is long and when you finish the prototype prices of the powervr3 (does it exist?) will have come down. Who knows a powervr4 may have appeared.
8. A tower with 3 5/15 bays would be enough, and a pair of 3 1/2 too. IMHO the A4000T (cbm or escom) look like a normal pc. I wouldn't try to make it as big as an A4kT. I have one and it takes too much space. Computers are getting smaller each day. Look at the barebones/mini-itx systems. I don't say you have to go to a really small form-factor, but very big towers are butt-ugly (a 1200 look better than my 4kT), weight a lot and give you little advantages. Make it of aluminium so it weights less and keep it as small as you can but keep space for three 5 1/4 devices and two 3 1/2 devices.
9. Under 500€ or the plain board and CPU if you want it to sell them well. You could put a G3 FX so you get altivec too. I would use an A1 or PegasosII CPU slot.
10. I don't know but as long as it's a number higher than 4000 it will be ok for me.
11. A bad one and it may lead to legal problems. What has to do NeXT with Amiga? nothing. If it was Apple at least they would have something similar...
12. Forget about using DC controllers. They are no longer manofactured. DC game compatibility is not a bad idea but remember than DCs used Gigaroms and you couldn't read with a normal CD drive. You could use pirate cds of course, but I can't see that as a good idea, so forget anything about DreamCast.
