I'm using Pololu 37D gear motors, and they've been OK so far -- but I've not really put any real load on them yet.
They also have smaller versions. I've also used their micro-metal gear motors, which are great for small things.http://www.pololu.com/catalog/category/22
For steering, I'd suggest starting with differential drive -- run one side faster than the other. A k a "tank steering." If that's not good enough (that's the mechanism that the Wild Thumper uses, for example,) then you can try building a suspension that can turn the wheel/motor combination. I'm doing that for my rover project, based on the "twin A frame" suspension.