The B4 and Centro (C4) are good, but a little expensive for swapping from rear motor to mid motor...... (170 quid for the centro kit, on top of buying the car in the first place!
http://www.cmldistribution.co.uk/cml...tId=0000006341 )
Some of the other cars on the market have all (or almost all) the parts you need to swap between mid and rear (Losi 22, DEX210 off the top of my head, probably some others).
If you are an ex 4wd man (i was) you might find the mid motor 2wd cars a bit more forgiving/fun, i hated my B4.1, but was mostly driving it on surfaces it really wasn't suited to (Astro and high grip carpet). Going to a Mid motor was an eye opener!
Looking at one that can be run Mid only (Cougar SV?) or mid AND rear might be a better idea.