There’s an Orgrimmar Market Night going on tonight, which will likely be attended by a heap of active Horde guilds. There’s no better night for having a look at Horde RP.
Off the top of my head, there’s half a dozen active mercenary/military/warband guilds, half a dozen blood elf exclusive guilds, and a couple of racially exclusive guilds for every other race.
But really, with levelling alts being so easy at the moment, I’d just spend twelve hours of free time levelling a Horde alt normally or six hours of free time levelling a Remix alt to play after the War Within comes out. No need to throw away money on a faction change.