Ebay is a good idea... You might be able to grab a great detector for a great price. I've grabbed detectors off ebay before and never had any problems... just make sure you get one in new or very good condition. also, ask the person why they're getting rid of it. then, just to be on the safe side, you might want to have it checked out, just to be 100% sure.
If you're going to get a water machine, look at some of the previous posts on this topic. My advice on buying big ticket items: don't sell yourself short and buy what you can afford NOW. figure out what you WANT and then save for it. If you have your heart set on an excalibur 1000, and get something else... you'll never be happy.
Excalibur is a great machine. I have a White's PI that I love in the surf. It is all going to be a matter of your personal preference... But whatever you get, get the best one available, or you wont be happy