Have you search the internet yet? I've seen dozens of websites with one or two metal detector projects. Some very simple, some with advanced features. Not really my thing, but I'd guess everybody has wished the had a metal detector at least once or twice...
Chemelec is a member here, and has a website full of various and interesting project, but I remembered he had quite a few metal detectors. Well worth a look, all are very well documented in great detail (should publish a few books...). Great site, plus you can get help here.
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Scroll down until you find the metal detectors, there are several. If nothing else its very informative.