Dear Fredrick,
Knowing that you already have a 5 weight I'm going to suggest that you look at at least a 7 weight. Personally I don't think you will see enough of a difference in "fishability" for bass by going up only one line weight.
There is a common theme amongst fly fishermen that it is best to split line weights as you add rods for different situations. ie. 4, 6, 8 weights, or 3, 5, and 7 weights. That is by no means a law, but it's a good general rule to be sure that each additional rod can serve an additional purpose.
With a budget of $ 300.00 or less you have literally dozens of quality fly rods to chose from. I'm going to stick with the brands I already suggested and say that within those brands and your price range you can find rods that run from the slow side of medium in action up to superfast. There are other brands in your price range that are also worth checking into, Orvis, Loomis, and Winston come to mind. They all have several lines of rods under $ 300.00.
Get out there and get casting. I'm sure you will find a rod that is suitable for the way you want to fish. One thing I will add is don't be afraid of buying a modern used rod. It's not like they wear out, and you can often get the top of the line used rod from a couple of years ago in excellent condition for about half of what it sold for new. Many fly shops take trades and sell rods on consignment for customers, and there is always E-bay.
Oh, and you don't need a boat to fish the Susquehanna with a fly rod, in fact for fly fishing a boat is an impediment. Somebody in the boat always has to worry about boat control while drifting and fishing time suffers accordingly for the captain. When the water conditions are right the Susky is easily wadeable, as are other large rivers like the Schuykill and the Delaware.
Regards,
Tim Murphy