New Owner with questions

Purchased a nice 2015 E4 with 2000 miles.
I will keep it at my Beach house and will have long period where it won’t be used. Do I need a trickle charger or if I leave it plugged in will it harm it.
Also where is a good site to by accessories for GEM’s covers, soft door, mirrors etc.
Can mirrors off hard doors be removed and place straight on frame.

You need a timer to turn on the charger for 8 hours every 2 weeks. (1 week is ok too)