Chris, I'll try to give your question an answer. But the question leaves alot of others be to asked. Most importantly, how much are you willing to spend? New or newer doesn't require as much cash to fix up. The older the boat, the potential for things needing to be replaced becomes greater. After all, you don't want to inherit someone else's problems, right? Of course, you can tell if someone has taken care of his boat, because pride of ownership shows and he'll probably have all his receipts for work preformed.
A good used boat will allow you to learn about it and save you some cash, which you will need for tackle and other stuff.
Some other questions. How big a boat (length) do you want? Aluminum or fiberglass? How big a motor? Inboard, outboard, or jet (don't know where you live ;D) 2 or 4 stroke?
Check the internet sites, tacke shop postings, newspaper and boat trader magazines to give you a good idea of what is out there and at what cost.
Once you find a boat you like, take the boat to a reputable shop. Pay the nominal fee to have the motor compresion tested and gone over to find any potential problems.
As for the salt water issue, there are boats made by Ranger, Triton, Skeeter that allow for both types of fishing, ie; freshwater and flats.
Anyway, I hope this helps, and good luck- JB