I frequent Passat (and by extension, A6) and BMW forums, as we have both a 5-series and Passat.
The key with any used European vehicle is finding a good independent mechanic who specializes in your car, and staying informed through owners' forums, which can save money and time. Basically you should be interested in your vehicle... an appliance it's not. Check out passatworld.com and bimmerfest.com.
With that said, most of these vehicles go through the same problems, so there shouldn't be anything unexpected. Our E39 (1997-2003 5-series) had a few niggles during its warranty period, then suddenly nothing went wrong for 40K miles (post-warranty, you're nowhere as anal). At 90K miles, the radiator developed a small leak, and I went for a complete cooling system overhaul (thermostat, water pump, radiator, fan clutch, belts, etc), as is customary. At 110K miles, the straight-six is still great, the transmission (GM Europe) is strong, and it drives better than most new cars out there. The suspension could go for an overhaul (bushings, tie rod ends, shocks, etc), which again, is typical for cars with high mileage.
My observations:
'06 Passats - these are nice cars, still the best midsize family sedans out there, but '06 was its first year and both cars are out-of-warranty; the newer Passats are more complicated... I'd buy them only if they were cheap
'03 530i - this is a very base model, with no leather, premium package, or sports package; $13K is too much
'03 A6 3.0 - this is FWD only, meaning questionable CVT transmission
I'd go for the Bimmer, but negotiate hard for discounts for future mods and maintenance.