1. both are popular options they offer the most flexibility, hacked game pads will buy you the cheapest cost per input for a little time soldering and some wire, 2nd only to keyboards which are really just a pita to hack and dont usually deliver on simultaneous inputs.
2. spinners are'nt use that much neigher are flight sticks..
your most common control is gonna be a 8 way, follows by a 4 way, then probably steering wheels, and after then every thing else.. depends on if there are any games out there that need something other then a 8 or 4 way that you gotta have..
i know there are a number of track ball games i like.. namely bowling.. golf aint to bad, and of course marble madness, rampart, etc.. although despite that i have no intention of my first CP having a track ball since it's a relative minority of games out there for it, but this is something you gotta decide for your self.
3. i would assume that 8 ways with restrictor would be as good as any 4 way only stick.. as it's really the restriction of motion that gives you a 4 way, but i can't say for sure.
as for buttons.. 2 sounds like a good number, i might shoot for 3 just for good measure..
i do know there is at least 1 game that uses a 4way and 4 buttons, chinese hero but i dont think there are many 4 ways that use more then 3.. or really even 2.