Well, buying in Ebay can be risky, as many have found on many forums There is no detector that does it all. If you are new to detecting, start with the ACE 250. Get at least one extra coil, maybe two. It is hot on gold, likes meteorites, coins etc. I would suggest you hunt beaches for coins first, to get you orientated to the hobby, machine-etc. Then, if you want to specialize in gold hunting, you can consider the Garrett Infinium or Sea Hunter Mark II. These last two are specialized detectors, and should never be purchased as a first time user due to their many complexities.
Buying new gets you a warranty. Buying used does not. Buying a major brand like Garrett gets you to forums with lots of experienced users, where you can get good advice. Buying cheap machines pretty well leaves you isolated.
http://www.garrett.com/hobby/hbby_products.htm