I can't say I have experience with the Nav unit, but I installed the Alpine w505. As far as the sound quality, it totally depends on 1)the aftermarket unit and 2)the installer's knowledge of setting up the outputs to communicate with the BMW amps/system.
With that said...you get what you pay for. A nav unit at $125 installed is simply unheard of, and I can only guess you will not be happy with it beyond the fact that you have a 7" display. One thing that always baffled me about some mods I see to certain vehicles is that someone spends 20, 30, 40 grand on a car, then skimps on the mods.
I guess my best advice would be to do your research a little more with a few different install shops, so that you better understand what you will be getting before you decide to go ahead with this.