Apply the KISS principle: Keep it Simple Stupid. Handing the property owner a 15-page legal document may only create 'push back' with him. Though I have some legal training, I would approach the property owner with a ' back of a napkin' type contract in this case. Cover the bases in your agreement: reasonable hours of entry, permission to bring in equipment, your agreement to replace the land into the condition you found it, % split (I'd OFFER, not ask for, 50%). I'd also show the landowner some goodies you may have previously found--but never divulge what your current target is. I'd also show the owner how your MD works (if a MD is part of the equipment you'll use).
You'll need a cover story-as to why you picked this person's property to search; obviously, don't mention the treasure you seek.
Many other 'pointers' I'm sure can be offered up by others in this Forum. I'll give you one more: Don't start to search in the 'bullseye' area of your search; start far away from that area. The owner may be watching and he may decide to go to that same area and search after you leave. He may also insist he go with you for a while--until he gets bored--then you can meander to your primary site.
Don....