Ah, you are right about the accuracy, haha. Resolution is indeed important!
Theres a free version for Alibre. The functions in it are good enough for basic designing, except the "assembly" function.. which I think is broken.
Here's what I cooked up using MLCAD. I've only used the basic lego technics components.. there were more advanced ones which I havent used before, so i'll leave that once I understand their functions.
This is the 6 wheel idea, with a basic rack & pinion system (lego style!) at both ends
*yawns* thats it for tonight!