It's still highly recommended to get separate rims just for the snows, regardless of whether you can find a place that will mount them on for you for free every year.
Everytime you mount or unmount a tire, the tire wall gets a bit of damage. Also, if the folks who do it aren't careful (and I've seen this happened), be ready for nicks and scratches on your nice BMW rims.
If you're going to pay so much for 17" snows anyway, IMHO there's no point to save the $$ on rims. If you're on a tight budget like me (or if the place you live is like Toronto where road salt is used during the winter), steel rims are a good choice as they are cheap and you don't have to worry about them. Of course, some people (like some of my friends) hate steel rims as they make the car look like crap. I just don't care because all cars look like crap in the winter anyway. I leave that decision up to you -- go for good looks and pay for alloy rims, or save money and get steel rims.
As for Blizzak. This is also my first BMW so I do not know. I have used them on my old Integra and they were amazing. However, be advised that only about 50% of the tread on the Blizzak is the actual "snow compound", meaning the tires are snow tires for only the first half of their life. Once you're down to about 50%, they become regular all-season's. So, if you can afford them or want some of the best snow tires, get them. I decided not to get Blizzak's on the car I'm driving right now because of this, and I'm pretty sure if my new 330ci does need snows I won't be getting Blizzak's either.