We are interested in 5 acres that has a 1971 mobile home on it. Our plan is to build a home on the property. My question is can we mortgage the property without including the mobile home?
You're essentially talking about a land purchase, as the home may not even be real property (if not legally attached to the land) and has little/no value when that old. I'd talk to your local bank about that, but in most cases a fairly sizable down payment is required for land purchases. Once the house is complete, obtaining a mortgage should be fairly straightforward (based on adequate credit, income, etc), the hard part is going to be finding someone to finance the construction and land purchase.
Mobile homes depreciate in a similar method as vehicles. They rarely increase in value. Thus lenders will not finance them like a stick built home. You can however get financing on the vacant land which would include the value of the improvements like well and septic.