You will want to get motor mounts from robo challenge too. Infact it might be worth getting wheels drill motors and the motor mounts from them, Unless your wanting to use the drill batts in your robot, But id advise getting a couple of nicad or nimh packs. Your going to need a charger and possibly a powersupply for the charger unless the charger comes with the 230-240v mains plug already. You can probably get everything you need minus some parts for your pneumatics system from robo challenge and technobots. Might aswell get the Sabertooth Dual 25A speed controller if you order bits from technobots as they sell them. And the DX5e is fine.
Basics:
Wheels - Robo Challenge
Motors - Robo Challenge - Screwfix - Argos
Motor Mounts - Robo Challenge
Wire and Connectors - Robo Challenge - Technobots
Link - Robo Challenge - make your own
Speed Controller - Technobots
Batts - Ebay - Vapextech
Charger (maybe a supply too) - Technobots - Ebay
LED - Technobots - Ebay - Maplins
And its worth putting an ad in the wanted section for some bits too. You might get some good deals.
You can shop around and get things slightly cheaper but I prefer to get as much from one place as possible, plus say you was getting afew bits from one place and find another place with the same thing a little cheaper it will probably end up costing the same because you have to pay another postage cost or whatever.
Really all robochallenge have to do is start stocking speedos and leds and they will be close to providing a full kit to start a robot. If they do start doing speedos they should be a uk distributor of the Robot Power speedos or something, Does anyone sell the Robot Power ones over here anymore? I prefer them over the Dimension engineering ones.

I wouldnt go for a flipper straight away unless you have a bit of experience with pneumatics.

wha? ehhhh bed time I think, Falling asleep typing.