no worries about the gfci breaker in your main panel, this is what i did for my installation. i found a breaker on sale so it was only about $20 more than the spa panel but it saved me the effort of mounting the panel, additional terminations, one less thing to go wrong.
yes, weatherproof boxes/covers are available. here is one from menards:
http://www.menards.com/main/p-1444451265330-c-6432.htm?tid=-7042359561445582654
the above is for a single-gang box. you might want to consider a two-gang box for the receptacle, makes the wiring easier (more room to work). you would need to get a two-gang cover though.
yes, weatherproof boxes/covers are available. here is one from menards:
http://www.menards.com/main/p-1444451265330-c-6432.htm?tid=-7042359561445582654
the above is for a single-gang box. you might want to consider a two-gang box for the receptacle, makes the wiring easier (more room to work). you would need to get a two-gang cover though.