The "s" usually means the car has been lowered a bit and better suspension for cornering has been included; sometimes sports seats will be installed.
To save some money, your city or town might have some one - service station - that specializes in BMW's only. I have three Bimmers, a 1994 318i, 1997 528i, and a 2002 330xi. I take my son's 318i into a service station in town that only services BMW's. It feels like your taking it to a BMW service department, but you save money in repairs because it isn't a BMW service department.
Some items cost more for repairs for a Bimmer, but not bizarrely more. So far, the only bizzare repair I paid for was actually cheaper, but it made me angry. I took my car in to get a slow leaked fixed in my front tire on my 330xi. The car was in for an hour and a half and the repair bill was $63 which I considered crazy. If I had of taken the car into "Joe's garage," it would have been repaired in 15 minutes and cost about $20.
I'm sure you have it in your area, but here in Vancouver BC the dealerships distribute pamphlets saying how much some of the service charges are to illustrate they aren't that bad, and these charges are "fixed" so the service department can't jack them up.
Be aware that a Bimmer is a more complex car than the average car so there is more to go wrong with it, especially if it has all the whistles and bells. But you know what, I predict you will love owning your new Bimmer.