A) You don't really need a 4 wheel drive. I'd recommend using a differential drive, since it's a lot easier to construct.
B) All 3 kinds would work actually, although a DC motor might be too fast (unless it's a geared motor or you gear it down) to control remotely properly.
C) Hmm.. I'm not very good with torque calculations.
So, I recently built a differential drive robot with a single castor, that carries around 3 lbs of weight. I used 2 standard servos originally, and the speed for that was ok, although torque was kind of low. However, I used really big wheels which reduces the torque. So, for a robot carrying 2.5 - 3 lbs, those standard servos should work fine, especially if you get decent sized wheels unlike me
If you're intending to drive it remotely, it's probably better to go for a lower speed, unless you're really good with remote control cars
Personally, I'm terrible at that so I have trouble controlling my robot even though it's not going very fast.