Budget will dictate the quality of detector you will get, buy the best you can afford...you will be very glad you did.
The less costly the detector, the less sophisticated it will be. The learning curve is very steep and regular practice is required. If you can spend more money you can get into a more automated machine which will flatten that curve somewhat. If you can spend about $400 you should be able to find a good used Minelab X-Terra 70 on Craigslist with a few accessories. Good headphones are a must.
For starting out you should learn on a round pan. I like the Garret Gravity Trap and they have a lifetime warranty. Once you master the round pan, try out the LeTrap Square pan. They are super fast.
You can get most everything here at a good price
http://www.armadillomining.com/