I've got the same car and had the same dilemma, last year I bought the parts!! started off with the lower section from a 1.8t (£25 ebay) About a week later I was having a look at an s4 avant, specifically the rear bumper and decided thats exactly what i needed, now, I have one of them lying in the loft beside the 1.8t section!! The s4 bumper cost considerably more, as I also needed parking sensors, and two mounting brackets from Audi.
I have arranged to have stainless back boxes made when i do the conversion, we did consider cutting the existing exit pipes and adding shaped tailpipes, a couple of reasons why I'm not going down this route, firstly I prefer the finished look of a complete section and secondly when I mentioned these changes to my insurance company they spat the dummy ("it does'nt matter if the changes have no effect the vehicles performance its still a modification to the manufactured spec!!" so, I might as well do the performance thing) Now I'm saving for a couple of other mods and will eventually insure a modified tdi.
Overall, I would'nt consider it a difficult alteration, there appear to be plenty of lower bumper sections about, just get one (in an ideal world, painted to your colour) and clip it on, you could cut the tailpipes and even buy a couple of those bolt on extension pipes (my Volvo t5 has such a thing as standard, fortunatly its not obvious, unless viewed from underneath) the painting will be the most expensive part, unless you know someone in the trade.
cheers
Iain
