Help with choosing motors.

Hello, I have a project in mind with my RPi(Posting in Arduino forums, lol.) and I simply need some motors to move the RPi and a couple more things(Servo, batteries,L982N, HR-SCR. It's basically a rip off from the starting robot on letsmakerobots. And sorry if I murdered the module names.). Here's my amateur scheme Forgot to ground the l982n to battery and turns out I'll need another battery pack or an UBEC for the servo because it draws more current than the RPi can provide. The battery pack doesn't need to be 12V, just needs to fit the requirements.
2 motors with a free-wheel in front basically. Now, I have no idea what motors you should get. I Googled and most people say that 6-12 DC motors are all you need. I'm going to be ordering online since I also need wheels. But, I have no idea of what motors to get. They all have different RPMs, different sizes and so on. Will I need gears or can I just control the PWMs on the Pi? I just have no idea where to start with motors. What do you guys recommend?

12V gear-motors with wheels attached are probably the simplest option.

The L298 is very inefficient at low voltage due to 2+V losses so 12V is better
than 6V.

Small DC motors run at 10,000 rpm or so, gear train is a requirement.