most piston seals can be found through local parts warehouses, but depending on wheather or not the caliper has been replaced before or not, this could be difficult. Bendix sells seals for their calipers, but if it's factory original, ??? E-mail me, and let me know, and I will see what I can find out. (I do brakes, steering and suspension for a living) As for removal, a set of mechanics or dental picks work best. Just be gentle, and be sure not to scratch or mar the caliper piston.
From Opel GT Source, of course. Or probably even your local Auto Supply store (NAPA or Bumper-to-Bumper). Sometimes they show up on ebay (I just bought a complete rebuild kit myself this week). And there are several other on-line choices (look at the links on this site).
As for hints, buy a good factory manual, and follow the instructions. But unless you have the proper "square" internal O-rings (VERY rare!), do NOT split the calipers when you re-build them. Just clean up the bores and pistons with fine emery cloth, and install the new fluid seals in the bores, and new dust boots over the pistons. If the pistons or bores can't be easily cleaned up with very fine emery cloth, they will need to be replaced. Keep the pistons matched with the bore it came from. Clean with alcohol ONLY (NO SOLVENTS!). Replace the flex hoses unless they are very new, even if they "look" OK, since they fall apart internally. And flush the old fluid out, and replace it with new clean DOT 3 or 4 (glycol-based) fluid. Only use DOT 5 (silicon-based) fluid if you have replaced ALL the rubber components and have thoroughly cleaned out the old.