1) Depends whether a 17" rim will fit over the brake calipers - that may depend whether they're BMW or after-market rims AND whether BMW list it as an approved size for your model - otherwise you'd need to declare it a modification to your insurers.

2) A 2nd set of alloys is better but is extra expense and an extra set of TPMS valves needs to be bought (unless it uses the simpler ABS system).

No comment on 3) or 4)

1, Using winter tyres will be fine as long as the circumference matches that of the 18's. There is a degree of difference allowed, but it is small, something like 3% +/-

2, If the tyre fitters you are using damage the rim removing the tyres then, a, they are liable to repair said damage, and, b, you should find a better fitter. (and by the way, you will obviously need another set of wheels of some description as your 17" winter tyres sure ain't gonna fit on your current 18" wheels!)

3, you're car will run perfectly fine without run flats (in fact the ride will probably be better with 'normal' tyres), the run flats are just to get round having a spare, or can of spray in tyre sealer and compressor.

4, Yes, it is perfectly fine to use after market alloys, so long as they have the correct PCD, offset (o/s) and centre bore (c/b)

To answer your questions in order :

1. 17 inch alloys MIGHT be ok. It depends if you have the MSport brakes fitted or not (blue brakes). Have a look inside the wheels if uncertain. If blue brakes, you need minimum 18" wheels.

2. I'd say yes, get another set of alloys. Swapping over tyres on the same wheels invites damage, and will cost a decent amount, both when putting the winters on and then when putting the summers on.

3. Any standard alloy. But buy a decent brand, rather than something bog cheap.

4. Yes, aftermarket alloys are fine for use, as long as they have the correct wheel stud pattern and offset.

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Now, I've got a 2013 RWD 325d MSport touring with blue brakes, my winter wheels are these, from mrwinterwheels :

www.mrwinterwheels.co.uk/18-bmw-3-series-f30-black...l shod with Pirelli Sottozero tyres.

Do note that you WILL need to buy TPMS valves as well, otherwise you'll have a permanent yellow warning on dash/idrive.

I'm actually going to be swapping over from my 19" summers to the winters this week.