300 is quite decent price for e92 install
i've installed individual audio (13 speakers) instead hifi in e92 and spent almost 20 hours and tired a little
so if the shop worked on bmw before it is more wise to ask them to do
but they should draw a scheme to show that they understand bmw audio system
the main culprits with e92/e93
1. since there is no rear door it is wise to remove front seats
doing this be careful with door plastic sill, seat is heavy
2. the routing of the cable to door harnesses, there not so much room under trims
3. headunit can be scratched by gearbox shaft during removal