I wouldn't touch any of the German makes, esp used ones.
Sounds to me like the the Brand cachet is important to you, based on your choices (Acura, Lexus, Audi).
You won't go wrong with the Lexus, but its kinda small and expensive, and the auto rags don't talk too kindly about the 250. they love the bigger 350 though, which is the only IS version I would be looking at.
As much as I like the Acura, I wouldn't choose the TSX. Its soon to be discontinued, and if you can ignore the badge, the Honda Accord Sport would be a better choice over the Acura TSX. It's a more modern design and its engine has Direct Injection, which the TSX does not. If you look at the Accord Sport vs the TSX on the dyno, you will see the Accord engine makes much more usable power in the lower rev ranges, where as the Acura, you really have to wring it out to get moving. The Accord Sport manual does 0-60 in 6.6 seconds, so its plenty quick. Has trick 18" wheels, heated seats, the best damn shifter and clutch in the business, and other niceties.
Why buy a used Acura, when you can get a brand new Honda Accord Sport manual for around $22 grand + tax + freight (Honda has a $2000 cash back offer, and dealers are discounting this car in the $1500-2000 range as well, which will bring you to the $22g mark)
Why buy somebodys used car with potential problems, when you can buy a new reliable car with warranty in the same price range.