Get you a 8 to 9 foot long 6 weight for now. You can manage the smaller trout flies alright, but still handle decent sized bugs and streamers for bass. BUT, you may as well plan on getting lots more rods. Fly fishing is like any other hobby. Once you decide you like it and start to figure it out, you'll go pick up a 7 or 8 weight for open water bass, then an 8 foot 5 weight for river trout, then a 6 or 7 weight with a slower action for streamers at night for trout (so you can tell when the rod is loaded without being able to see). Then you'll have to have a 6.5 to 7.5 foot 4 or 5 weight for small stream bass and panfish, and maybe pond bluegill. Then you'll develop a soft spot for split cane rods and have to start the process all over again with cane. And woe unto you if you decide that you like something such as steelhead or salmon or saltwater flats...
But, you're already a fisherman, obviously, so you already understand this progression! Enjoy the ride!
P.S. And then you'll read somewhere how much fly fishing resembles the asthetic of traditional archery, so you'll buy an old recurve bow. Then decide to start making your own bows out of hackberry trees behind the house...