First of all, welcome to the forum!
It sounds like you mainly want to fish for largemouth, in which case I'd recommend getting a fast or medium fast action 6 or 7 weight to start out. I think it will be better to get a rod specifically for your main species so you can learn how to flyfish with that rather than trying to find one rod for all the species you might
If you enjoy fly fishing for largemouth, then you can look at getting a saltwater outfit in 9 or 10 weight. You can get a very good largemouth rod for around $100 and a cheap reel ($35) to use with it, while a saltwater outfit will run you in the hundreds of $$$, mainly because you'll need a solid drag in the salt while you don't really need any drag in freshwater.