I had a blackview mounted professionally, also using the magic box (power magic pro) that in my case is hidden in the little cubbyhole just above the ODB2 connector. Not a single visible wire, so if your techie, knows what he is doing, should be allright.
And yes the magic box is connected to an always-on fuse and checks voltage to see if it does not drain the battery too far.
Your mounting price is dirt cheap I must say. The power magic box itself is about 20 GBP
V40 D4 Summum R-Design Caspian Blue Metallic, with a tad of Rica magic applied