Read the instruction manual first ...
accuracy and setteling time for SPECIFIED POSITION READING
Battery Life
Is it REQUIRED to be Waterproof
Is interface to laptop PC a requirement (now or in future)
Advanced cost usually adds maping features and color display -- what do you want to really do -- follow its directions or locate and log where you are --- and to what precision -- most are going to be 10 to 20 FT in reasonable time.
Ifinder units at sporting goods stores are under $100 and work OK (to get you in the area). Surveying type accuracy costs big bucks and usually requires 6 channels.
Always carry extra batteries.
Go buy golf T's (pack of 250) and drill small hole to insert wire-brad into end (drill press and flip open jig)... make a bunch and use them as markers. Believe it or not, works pretty good if you put pattern in ground to mark location for future investigation (they are almost invisible, but give marker pattern to metal detector, can use copper wire or brads if non-metal mode wanted). Wood dowels or short 1x2 (sharpened point) with nails inserted in top will also work good, but may require hammer to drive into ground. Top may be painted, if you want a visible marker. Home Depot has 10 and 12 inch galvenized spikes and if you look closely there are sometimes some 6 to 8 inch nails (may get a little flack for using those though, not to mention next metal detectorist digging them up).
Regardless, take a compass (liquid filled) and make a bearing map to landmarks to get you to right starting point. Drag out old boy scout manual on map making, or military survival maping skills. Topo maps are available on web or use Google Earth / Microsoft Terraserver. Delux GPS units allow downloading of topo maps into unit, to display trails, etc. Check out National Gographic Mapping Software Packages -- (Half-Price Books Sometimes, REI Always)
