TL,
I agree with what Goos has to say. Look for a river runner with play capabilites. For beginners, it's all about comfort. Each manufacturer has their own "fit". You might find that you'll be loyal to a specific boat manufacturer because you like their fit. I've always been a big fan of Wavesport's comfortable and easily adjustable outfitting. I don't think you can go wrong with a used EZ or Ace, if you fiind their boats comfy. Lot's of those available in gearswap at great prices.
Everyone says to demo, demo, demo. But in reality, demoing a range of boats on water is really difficult, especially early in the season. Sometimes you've just gotta' go with your gut instinct, talk to a lot of people about boat design, and sit in a lot of boats. I think this is true for a beginner when you'll not be as receptive to minute design issues, such as hull length, rocker and chime depth. I think once you sit in a boat, you'll know if it's right for you.
If you buy a used boat, scrutinize the boat for cracks. If in any doubt, ask the seller if you can fill it with water from a garden hose to look for leaks. Scratches and small gouges are to be expected. Make sure all the bolts are in place. Don't ever buy a boat that's been stored outside and exposed to the elements (look for leaves and debris inside as a tell tale sign). Offer the seller $10 for a trial run just to make sure before you buy it. You can expect to pay between $350-500 for a recent used boat in great shape.
Good luck,
Dav