Having owned a B5 S4 for 13 years and done over 100k, my advice is that don't get put off by high mileage. Audis are built well and to last, and so far it seems to me that the B8 has been designed and built better than the B5, with a few exceptions. The s-tronic gearbox you know about. The V6T engine is also good perhaps apart from the timing chains which can need retensioning at high mileage, and the small risk of an intercooler leaking. Other issues such as the waterpump and PCV are relatively minor.
As you realised, a 2010 B8 with evidence of s-tronic gearbox problems is to be avoided. But not all early gearboxes go bad. Unfortunately it is not possible to tell if the 2011 car has the 1st or 2nd generation gearbox without investigating. However even if its the 2nd, they weren't trouble-free and so it would be worth getting a warranty which covers the gearbox. If I were you I'd also want to know why the original engine needed to be replaced, just to make sure that it doesn't cast suspicion on the gearbox or indicate a problem (e.g. collision repair). Other than that if its a well specced car at a good price, then as long as you allow for having to meet some repairs in the future (e.g. wheel bearings, bushes & gaiters, steering and suspension components etc.), the 120K shouldn't be a reason not to buy.