I printed all parts in PLA with a 0.2mm layer height, 15% infill, 2 solid layers top and bottom, 1 perimeter. The longest part is about 5.5" (140mm) so you do not need a massive printer to print this.
Print the following quantities:
The lower deck easily accomodates a 5 cell 4/5A NiMH battery pack. Im using 2100mAH.
The top deck mounting hole pattern is layed out for a standard Arduino board, and the legs are made to fit the Pololu 120:1 plastic right angle gearbox
The ball bearings for the rollers are 3x7x3mm which can be purchased in packs very inexpensively from Hobbyking
All mounting hardware are 4-40x1/4" socket cap screws avaible from Microfasteners. The only exception are the 2-56x1" socket cap screws and the associated lock nuts for mounting the gearboxes
Assembly is very straight forward, and should be self explanatory from the photos and renderings. For wheel assembly, you will want to first bolt the hub to face of the wheel_2 part. Then, you will need to attach the hub/wheel assembly to the gearbox output shaft. Finally, the wheel_1 part can be bolted to wheel_2. The rollers can be installed at any time after you insert the ball bearing. However, you will probably want to sand or file the inner surface of the wheels that face the rollers, so as to reduce friction between the roller and the wheel. Doing this before you assemble the wheel is easier. The bearings should be a press fit into the rollers - if they are too loose or too tight, fine tune by adjusting the filament diameter in your slicing software. Also, be sure not to overtighten any of the screws, and they are only threading into the plastic.