Gopi,
It's been quite awhile, but I built one based on the 3Pi. I basically built a new top-board, and it was Microchip PIC based. If I had to re-do, or do a do-over, I would build something based on an STM32. They are pretty easy to write code for, but you can have some serious computing power. The biggest problem I ran into was I was speed limited by compute. The best strategy is to use your first two runs to map out the maze, and then use a flood-fill algorithm to compute the optimal path to the middle, and do your final run fast.
Anyway...Mechanically I thought the 3pi was a good pre-made base to start with. The dimensions are pretty optimal for micromouse.
-Ted