You can get those RadioLink Tx/Rx combos for £25, but you pay or what you get....

You can get DSM2 Rxs on eBay for around £6, just search for Orange Rx.

Sabertooth controllers have current limiting built in, so any spikes and they cut the power, in a 13.6kg bot the 12a model can give some juddery output when you're being aggressive, but since your robot is in 2 halves it should be fine.