I know on the 8P A3 a lot of guys on the 8P forum asked the question about seeing if BOSE could be fitted to there A3 that didn't have it. It simply wasn't cost effective as it meant rewiring the cars sounds system etc.
Mono Pur - you might be able to do if you can buy the door cards with Mono Pur already fitted. I know there is more of it in the car but that's the best idea I can come up with.
Same applies to the seats. Expensive but could be done I'm sure.
Almost any thing can be retrofitted at a cost. But when you think that the radar sensor alone as a part is listed at nearly £1000 it's certainly not going to be cheap. As far as I know none of the 'retrofitters' list ACC retrofit as an option.
Why did you not order it with the car in the first place?
Being already on my 9th A3 I have learned over the years that no dealer is ever going to have the car I want in stock and no second-hand A3 is going to have the spec that I want so I have to plan ahead and take the delivery period for a factory build into account.