I use the Garmin 60CSx in the field with great results. It has plenty of features, yet it's fairly straight forward to use right out of the box. At most it would only take you a day or so playing with it to learn, not so much how to use it, but where the different functions are located. Essentially, which sequence of buttons to press.
I purchased Garmin's Mapsource United States Topo Maps and a decent sized memory card, which allows me to hold the topo maps of nearly half the United States.
For mapping my treasure hunts and other outdoor adventures, such as any hikes I take or little hidden fishing spots I come across, I use National Geographics TOPO 4.0 (New England Version). I like the NG Topo software for both the detail of their maps, which go down to 1:24,000, and for the ease at which you can transfer waypoints and routes between the software and your GPS.
I will sometimes use google earth to mark the location of something I've found, particulary if I want to share that spot with a friend, but I don't think that's what your looking for in this post. It seems, though I may be wrong, that you want something with more mapping capability. Software that allows you to print out your own maps with your own tracks and trails indicated as you make progress detecting the site. Plus at the same time something that integrates seamlessly with your GPS so you can record, track, modify, view info on your GPS unit.
Your really talking about two components to get your desired results, a good GPS unit with mapping capabilities and topo/mapping software to integrate with your GPS and give you a visual representation of your efforts in the field. For it to work, as I think you want it to, you need to look at both pieces.
The products I've mentioned will definitely do all that and more, but combined they will set you back at least $500 give or take, when all is said and done.
I would venture a safe guess that you could accomplish your goal and come in under that priceline if you do your homework. If that amount of money is reasonably within your budget, I'd have absolutely no reservations about recommending the products I've mentioned.
My thoughts on GPS units, if you don't already have one, is buy up. Not too many people complain about having too many features but you know how it is when you buy something and later wished you went for a little something more. I have several Garmin products, hand-helds, car units, etc. and they all work perfectly and have never disappointed me on quality.
Johnny Cache
https://buy.garmin.com/shop/shop.do?cID=145
http://maps.nationalgeographic.com/...O! Outdoor Recreation Software/?cid=135&tid=3