you can make any caliper fit if you have the right bracket.
most audi (B5, B6 and B7 anyway) have lug mounted calipers, you either have to have a bracket to convert one lug pattern to yours or a bracket with perpendicular mounts to convert radial mounts to your lug mounts.
Brembo make the most widely available OEM upgrades, (subaru, mitsubishi, porsche etc all run Brembos.
You need to match a caliper (4 pot or 6 pot if you are upgrading) to an OEM or Mercedes disc. That way you have the right stud pattern and the discs are easily available. check the width of disc that is recommended for that caliper, numerous options available.
If you are going to do porsche go for mono block 996 and upwards 4 or 6 pots.
The "S2" brakes you refer to are 2 peice castings, not a problem in itself but the pad carriers corrode btween plate and caliper causing a pad squeeze and ulimately a binding. pain to fix and unecessary given how many otions you have for better parts.
You can tell the 993 and "s2" calipers as they have a wire pad retaining spring on the top (outside) of the caliper. later ones have a flat spring filling the gap.