I would recommend not replacing your MC with a used one, I would recommend going to somewhere like AutoZone, Checker Auto, Advance Auto, Kragen, Shucks, Napa (little high priced) or any other local auto parts shop and get a rebuilt or remanufactured Master Cylinder. As far as I have been told, the ABS in our generation is very unreliable. I believe it ias made by Bosch? (Someone will probably correct me) Anyways, the Master Cylinder should only be in the range of $60 as far as I know.
But before we get rushing off into replace this, replace that, what are your symptoms? I am guessing the prerequisite for this posting is that your ABS light is on. Other than that, is your pedal mushy? Are you leaking brake fluid from your Master Cylinder? Leaking fluid from either brake caliper or drum?
Now I don't know a whole lot about the ABS problems with our gen because I have standard brakes, so I really don't know what part of these brake systems are so unreliable, or anything about ABS brakes for that matter. But to help out the gearheads, please describe your problems and symptoms in great detail, and possibly post pictures if applicable.
Good luck with your brake woes!