Generally speaking glass is like most other parts for your car i.e. you pay a lot more for a part if it comes from the main dealer, even thuogh the same part from the same manufacturer is available elsewhere cheaper, but without the BMW badge. I'm not sure what price this screen should be , but you could get a price for a screen and a moulding from BMW and then get a "fit only" price from autoglass it's possible that this could be cheaper.
With regard to your mirror,sat nav and tv, none of these should cause any problems.
Whoever fits your windscreen needs to know what they are doing as a badly fitted screen can lead to all sorts of problems, from undermining the overall structural rigidity of the vehicle, either through corrosion (which will eventually lead to bond failure) or the use of non OEM spec urethane, to simple leaks or wind noise, not to mention the damage that can be done in removing the old screen e.g. paint work, dash , headlining. So basically keep an eye on the fitter and hope he is into cars because some of them just don't give a fcuk.
hope this helps, Don