I have the Bottomline 1200 and have been very happy with it. Cabella's sells this model for less than $100. It's cheap, easy to install, and works great. Like G-Man, I find it best for finding structure though it does mark fish. I suggest you buy the rechargible battery that Cabella's sells if you get one that requires a 12 volt. I don't know what the new models require, but when I bought mine last year I needed a 12v.
Definitely get a fishfinder. Knowing structure and water depths is so important. I couldn't imagine fishing without one.