Southwest RC have the RTR DEX210 with free MM4 conversion kit:
http://www.oople.com/forums/showthread.php?t=134013
As said, the Kit version comes with all the bits to setup as RM3/4 & MM3/4 whereas the RTR comes prebuilt to RM3. The conversion costs about £10-12 though so not all that bad.
Personally I bought the RTR. It cost me £145. The ESC & Motor is Speed Passion so a fairly good brand and the ESC takes upto a 9.5T motor. I guess it depends on how much you plan on spending on the hobby, the RTR will get you going then you can replace parts as and when you need/want to, all you need is a battery & charger. The kit will mean you'll need to purchase ESC/Motor/Radio gear plus a few other bits (assuming you dont have these already) before it can be run but depending on what buy you'll be looking at anywhere from £50+ for a decent ESC, £30+ for a motor, Radio gear can be had for about £20 but can pay upwards of £50+ for a half decent one.