[Aliendex]

1. You can choose whatever downpipes you want, they are all almost the same, coming from china manufacturer.
I bought mine from Aliexpress, they were 250€ or about 300$, fits perfectly and are on the car for more than a year now.
Don't buy M6 downpipes that are "650i like" with the expansion joint in between, they usually cracks and the joint is not required in our exhaust system.
Buy standard downpipes and add on top of that 50$ to wrap them with titanium exhaust heat wrap. The sound will be less hoarse, a pitch toned down (i can confirm that because my DPs were initially installed by me without the heat-wrap, then redone the work and feels the sound difference)

2. BM3 is really versatile. We don't have most of the fancy options available for the other engines but it's a safe tune and you have the ability to do some stuff by yourself.

4. Requirements are upon the stage of the tune you would choose.
For stage 1 you don't need anything, for stage 2 you need downpipes. Also true for the octane rating you choose in the tune.
When going stage 2 some people suggest to change the spark plugs in favour of the 1-step colder NGK SILZKBRD8DS (97506) gapped at 0.022" (0,55mm) but it's not strictly necessary.
(I have mine gapped and ready to be installed, but as said before my car is on stage 2 93oct for about a year still with the stock Bosch pre-gapped spark plug)

5. Yeah, you can turn it on/off in each map of BM3, even your stock tune