I wouldn't buy stainless bolts- you might not be fitting sufficiently strong items. If the existing bolts are that bad, which is often unlikely, just replace with the standard parts from audi/Pete Reeve/just about anywhere. They've lasted several decades so far.
have a look through-
http://audi.7zap.com/en/rdw/
for the part number
Find the right car, part number.
As for getting old bolts out, there is a tendency for the slightly exposed thread (where it passes through the mounting face) to get a decent bit of rust on it if it's never been released. What I find helps is to remove everything around it, like discs, calliper, etc, get in with a wire brush, and clean it up as best you can. Undo the bolt slightly, ˝ to 1 turn, tighten again lightly. Then another clean up, squirt of penetrating oil, and come back after a cup of tea.
Doh! Too slow typing.
As for the callipers- I'd squeeze the pistons home, and partially push them out with brake pedal pressure, several times, against a block of wood held in the calliper. if they move freely, don't get excited. You could put a new dust seal on.
With all of the Black-friday/stick-it-to-em-monday stuff, Bigg red are offering discounts. So if it came to it, you might get a good deal.