A couple of pointers:
1. Don't use a DC motor for your first robot, start with a continuous servo (modify acc. to tutorials here or buy ready). Why? Because you can't just connect a DC-motor to your circuit anyway you please, this will in worst case kill your microcontroller as DC motors generate lots of severe electronic disturbance. To connect a motor you need a motor controller, like an H-bridge. Servos are much easier and works great for alot of cases probably including yours.
2. Your frame is depending on the wheels dimensions, the length between the weels and so on. Either you choose a frame and then try to find wheels etc to go with that frame, otherwise find the wheels first and then try to create or find a frame to go with that setup.
3. Keep it simple. Ok, so your friends won't be too impressed with your three month robot project with a bot avoiding obstacles, but that is because they lack the knowledge of robotics. If you are building it from scratch as it seems, it won't be plug n play...
