Rear Calliper option

[J4CKO]

I have used a Trek 8000 as a winter commuter, just getting it ready and it needed a new chain, cassette and front big ring, cost me £80 odd quick, them I relaied the rear caliper had a piston stuck and there seems to chance of freeing it.

So, need to replace the back caliper, it doesnt have to be fancy, just so I have a rear brake, can I use a mechanical one ? it has Avid Elixir R's on it which were great, so any suggestions for a direct swap that isnt too expensive, dont need ultimate stopping power as it doesnt see any mountains.