The single most important part of any metal detector is the coil assembly. A PIC is usally only used to make 'fancy' displays of the resultant signal but you can't make a silk purse from a sows ear - in other words, without a decent coil/detection system using a PIC won't add any additional advantages.

Elektor magazine, November 1981, has a good (very, very good) metal detector circuit (with pcb layout etc) if you can get a back issue - my pdf of the article runs to around 8.5Mb in size.