Basically anything can be done, it just costs more!
19's with say 45 profile tyres (you said medium) will increase the rolling radius so you realise that your speedo will be useless!
Also it may mean losing the inner plastic arches. Plus you will have to check all the clearances.
As for cost, well think of a number! As you will be doing all 4 arches and that means 4 panels to respray at least, so I'd say anything from £500 to £1000.
Why cant you make do with 17's like the majority? Being different is a good thing but this seems to be taking it to extremes.
As for the Audi bumper you will have to check measure one or get some dimensions. If its narrower forget it! If its wider then it will have to be cut and joined in the middle and also new mounting points created.
Seems a lot of work when there are loads of body kits out there!