You can start by reading multiple posts around this forum on the various detectors of your interest. In my short experience of detecting, I have learned it is all about WHERE you plan to detect, which dictates what type of detector you should buy. So far for me, Beach detecting, = Excal, or Sov. GT. But for inland soil that is not beach sand, that = a whole different type of detector "speciality". In other words, pick the best type of detector, for the location you will be using it in. It makes a big difference. A good beach detector, will not necessarily be a good / best hard ground detector. Prospecting / gold detecting = a completely different type of detector. I bought a inexpensive Used 15 year old Whites 6000 DI Pro SL for my first detector for $250.00. It work's fantastic, for dry sand, and not so trashed up Dirt, but will not work well in wet sand, or highly trashed areas. It will not do well at all for Gold prospecting. Whites has fantastic customer service, Just call and ask them. Minelab in U.S., Good luck! (Expect to be told it will take 2 days for them to answer your question, then they don't call you back at all!) Buy used, cheap, first. Then save for the "Best for you" detector after you have learned just what that is. Or just spring for the $500.00+ for your best choice used detector right away. Just jump in and try it, it's a fun hobby, but don't think you will get rich doing it. You will be lucky to pay for the detector with your finds, and if your trying to get rich, it takes the fun out of it, and turns it into a very LOW paying job!
Check the adds on this forum here: http://www.findmall.com/list.php?44 Which is probably way better than EBay.
Good luck, it is a hard choice to make, that requires hands on experience, to figure out which is "best" for YOU!
Beachnut