You could buy a trukk and use those wheels. The trukk won't look that bad with just 4 wheels.
Another suggestion would be to visit
http://www.zinge.co.uk/ who has a great selection of wheels and vehicle parts you can easily use.
He has his own thread in the News section of Dakka too.
Regarding the deff-rolla, both my Wagons have the
GW set and I simply love it.
I magnetized both the ram and the deff rolla and used the Killkannon that comes along with the Rolla.
That way I can either run it as Killwagon with the ram, or as Transport with the Rolla