I looked at the ATTINY85 for the brain but I'm concerned that if I use 2 lines to PWM the motors I only have 4 inputs left for the line sensor. Is that enough inputs?
I recently made plans for something similar, but with a PIC10 (6 pins in a SOT-23 package) - but it won't be 'till I can lay my paws on a pair of pager motors (got a handfull, but all different).
As long as you don't plan anything fancy, you have plenty of lines, some 'bots use only two line detectors (some by detecting one edge of the line*), so you could replicate that and have two pins for other purposes.
* One sensor (say the left) see the line and the other see the background. Now if the left sensor senses the background, you turn left until it see the line again and vv. It will probably wiggle its tail a bit, so better give it a girly name