Also: is it possible to swap front and rear callipers? I've got a working deore front calliper kicking around in the parts bin that I could swap out. Would bring the cost down to new discs + pads + bleed kit.
Got a photo of the caliper with the rotor? Give the rotor a wipe with your finger and see if there's any oil on it also check around pistons. If there's noticeable amount on either then it's probably a leak.
The disc and pads could be saved depending on the amount of wear on them.
Might be worth a punt.
Also: is it possible to swap front and rear callipers? I've got a working deore front calliper kicking around in the parts bin that I could swap out. Would bring the cost down to new discs + pads + bleed kit.