Welcome to ribnet!
Assuming you have a "modern" licence, in round numbers the absolute max is 3tons for the whole rig, max for the trailer will depend o nthe car. Unbraked max is usually half the Kerbweight or 750Kg, whichever is smaller.
Storage in Edinburgh area I'll leave to the locals....
Car size? - Much debate, but FWIW, I tow a 5m rib on a braked trailer with a Focus. (I could get away with an unbraked, but I decided to "future proof" the boat against my next car(s) being lighter kerbweight. At that sort of length the Fiesta / Corsa sized cars are likel not going to be much use, but a Focus / Astra sizze will probably be OK.
I would guess a generic 5m rib will be somewhere between 500 - 750KG + trailer. Mine is "only" 430Kg, because it is so old the transom can only hack a 60Hp /110KG of engine, so I have a nice light Clamshell on the transom. If you are buying new(er) , the hulls are generlaly built heavier to take heavier 4- stroke engines. Bottom line is - check your data before purchase!
As for courses, I think SPR on here does courses on the Forth, also Port Edgar I think run them. Have a google - there are others out there, and if you can be bothered driving to the Clyde or the Tay there are lots to choose from.