For locating wires you'd be better off with a wire tracer, not a metal detector. I borrow my irrigation guys whenever I need to repair a dog fence, but haven't needed one enough to justify my own.
Are you sure you want to get involved with locating property corners? I worked a year or so for a survery company and will say that it is not easy. You have no idea how many metal pipes we found that WERE NOT the property corners. You really should leave that up to a professional survery company as the liability or consequences could be expensive. If a job is that critical and you need to locate them, I would put in the expense of a survey company into the contract.
As I remember, the metal detector we used was around $700. They are not all that easy to use, and require some skill getting use to. Pipes buried under the ground a foot or two don't make that much noise.
We used something like this when I worked for the surveyor:
http://www.measureandlevel.com/produ...gnetic_locator
For a wire tracer, the Progressive Electronics model 521 seems to be a norm for guys around here. It works great finding broken dog fence wires and low voltage faults. Also handy for locating utilities that don't get marked out. Think they are in the 1k range.