I'm inclined to say go for the 18v drill setup, purely to be different. Though many (ourselves included) start out with four drills overvolted. Done right it makes for a nice powerful machine, but no matter what you do, it will be dogged by reliability problems. Reliability will be greatly improved without overvolting.

Can you get a picture/any manufacturing numbers from the motors? Chaindrive is perfectly possible, but if the motors are fast-running (quite likely unless they're really quite big), you get into needing two or more stage reductions for it to have the power to drive the machine, and that gets pretty complicated (and heavy). Single stage means, yes, from the motor directly to wheel.

In short, if you can, pictures please!