I usually get by with undoing the two bolts and wriggling the caliper off the disc. Free the pads, then you can get in there with an old toothbrush and WD40 and I normally find that, with a bit of pushing pistons in and out, that I can get it moving smoothly again without actually removing pistons.. this saves the hassle of bleeding air out of the system.
Replacement (OE) pistons and seals looks like about £45, seals alone a good bit cheaper.
A blue spot will be around £50 s/h, of course you'll need pads too... and since it's bound to be a bit of unknown quality, you'll probably want to change the seals on it. Advantage is, of course, you can do all that work in advance without having the bike out of action.