You dont need to warn the installers of anything becuase you are going to install it yourself. It is easier than changing the oil. Going to a shop is huge waste of money. I emphasize HUGE waste. We're talking $50 minium parts+labor, when in reality it would cost you under 10.
All you have to do is unscrew two screws, unplug a harness, use butt connectors to connect the clearly labled wires on your new HUs harness to an adapter (available online or from most install shops), slide in a sleeve with a trim ring behind it, bend the tabs to secure the sleeve, plug in the new harness, slide in the HU, attach faceplate. If you buy a HU from crutchfield the trim ring and adapter are included free, as well as pictoral install instructions.
You will not screw anything up, i dont know what high/low button you speak of, but most likely yes it will.
Alpine, Pioneer, and Panasonic all make good HUs.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>