Here's a consequence of using steppers.
Example: You have a 7.5 degree per step so you get 48 steps. With a uc, it's pretty easy to go halfstep, so that gives you 96, let's say 100 steps.
At a 100 mm wheel diameter, you have 314 mm circumference, so at each step, your robot will leap 3 mm. If you run at a high speed, the leaping won't be so obvious, but at low speed, it will appear to jump instead of rolling.
So one thing you'll need for your steppers is a high number of steps.
If that number of steps still won't be enough, you might need a microstep driver (it can be done on a uc, but it's a real pain).
Finally, if you have steppers, on a flat surface you pretty much have odometry. However, it's hard to know if the motors actually made a step. You can check the induced current in the opposing coils, or you can add encoders. By only a combination of stepper + encoders, you can map out the environment (and do collision "reaction") in a simple way: send a few steps, if the encoders jump back and forward by a similar amount, you're not moving, we need to change direction.
All steppers are reversible, all you need to do is reverse the sequence.
Using steppers you'll get a good holding torque, that is, your robot might be able to sit on an angled plane without requiring power to the motors.
A disadvantage of steppers is that they require a complex drive mechanism for efficiency (commanding one in bipolar, you'll need two h bridges, that's four h bridges for two wheels). The are ICs for this job, though.
As for buying them, I'd definitely say ebay. You can get really nice deals there.
nanotec.de has the real stuff, if you have the money for it.
edit: also, steppers aren't very speedy (i noticed the rpm just now). From my experience, to reach their maximum speed, they require some acceleration. it's an odd type of acceleration, meaning they can get to full speed in a few tenths of ms, but they can't get there instantly. also remember that the higher the speed, the lower the torque. plus, if you go too fast, when they stop, the steppers will have to counter the robot's momentum, so the wheels might drift a little, so you'll have to account for acceleration and deceleration too in your final firmware