Hello Thaddeus, it certainly is an odd combination for your TT however the colour looks amazing! The e39 is a cracking BMW and no surprise it's a keeper! Best 5 series in my opinion :)
The normal headlights are more than adequate in my opinion.
With any car you're looking at, make sure all electronic items work as some of them can be expensive to fix. This includes the air con and headlights. If the car has xenons then make sure all likes work as bulbs can cost £180 approx each.
Also as you are wanting a car with Quattro make sure it works correctly. The system can be sensitive and can even be damaged by uneven tyre wear. So make sure the tyres have the same wear on the same axle and ideally they should be the same make.
If you are looking at a remap then you'll need to make sure the car is in good health. Also you'll have to contact your insurance company to let them know of the remap. (Sorry if I'm stating the obvious). It isn't just a case of what the engine can cope with but what the gearbox can cope with.
I've found this YouTube video which may help:
Also if you want Quattro and more power then it might be worth you looking at the a6 c6. Petrol engines are 2.0, 2.4, 3.2 and are all available with a manual gearbox and Quattro. this excludes the s6 and rs6 which runs on a Lamborghini v10 and is auto.
Hope I've helped :)
Cheers
Steve