Hi pugster,
For a simple feather weight I would recommend:
-2 Argos 12v cordless drill motors (12.50 each)
-use the 12v batteries from the cordless drills
-2 blue marine escs off eBay ( about £25 I think)
- 2 100mm Robo-challenge blue wheels
- giantcod transmitter and receiver (£25)
- v-tail mixer off eBay (£5?)
- 10mm hdpe armour
-5 mm hdpe base

Probably looking at a minimum of £120