Safest way: Drive up to the Montana Family) North-East of the map, left of Meth Dealer. There is a cashpoint there. You can buy the license there. I've personally never seen anyone around when I was a rebel, so I'd recommend that. Good luck fighting the police!
Well, having a reb license and guns really only adds protection/force on the island, it does not really do anything else. Well, if you get good gear and a couple of other guys you can take on the HM Treasury. You can ofcourse also rob gas stations and banks, but you could do this before with just a handgun. You can also rob big convoys of drug people. Make them stop, kill them/take hostage (with roleplay ofc) and sell their drugs. The options are endless!