The rear brakes require a tool to spin the piston in to make it retract. Most likely if you buy a reman caliper, it will already be spun in all the way.
If you are replacing a single caliper, and you are quick about swapping the lines, you can usually get away with bleeding that one caliper only.
It's not a bad idea to bleed all 4 wheels considering the fluid might be almost 10 years old.
Pick up some speed bleeders at the parts store, they will run about $20 but they will make your job so much more easier. SImply swap the stock bleeders with them and attach a hose. Crack the bleeder and pump away. No need to close the bleeder after each pump and then reopen.