I agee with LT, the books are the best place to start. Buy, read and understand both books. The $100 or so you spend on them will easily be recooped on your first rebuild.
The XS first stage is basically a Scubapro MK-5 clone. The XS manuals are on line somewhere, I have a copy but don't have a clue where I got them. I am a manual junkie and never pass up downloading one.
A good second step would be to find a used MK-5 for a few dollars and learn to rebuild it. All the mechanics are the same and there are a lot of MK-5 fans here that can help you along. You may end up finding that you like a MK-5 with a 109 second better to boot. The second is a pretty standard design, almost all of the balanced, adjustable seconds on the market today are very close in design. If you can find a used balanced second, the service basics are the same.
For that reg in addition to normal bench tools, you will need a pin spanner and a bullet tool. Both are fairly inexpensive and the bullet tool can me easily home made.