[Andrew Schreiber]

Modding an RC car to have Mecanum wheels would be very difficult. Also unless you are planning on making your own Mecanum wheels chances are you will have difficulty finding ones that are small.

That being said, the simplest way to build an rc Mecanum would be either the vex kits or the mindstorm kits. I think Tytus has some designs out for either omnis or Mecanums at that scale. I also recall seeing a lego Mecanum wheel. If you decide to go this route I will see if I can find them again for you.

Now, to mod an RC car would require:
1) Mecanum Wheels (see earlier)
2) Minimally a 4 channel transmitter and receiver (most small cars have 2 channels)
3) 4 speed controllers
4) 4 motors (small, low current draw unless you want to spend a lot on speed controllers)

None of these are particularly hard to get, it may end up costing you more than you want to spend however.