Hi, some wild stuff here, one the cell phone rental: The 50% Rule does not apply to such rental income, it's a ground rent, not a building with tenants in it.
You need to assessthe value of the leased grounds and the lease as if it were a seperate piece of property and then assess the value of the other peoperty as a rental.
Having granite everything will probably not be common for the area. If I were you, that is one of the first conversations I would have with him, does he realize that he has "overbuilt" his property for the area?
As to financing. Talk to your bank, while 20% is generally required for non-owner occupied, they can go 10% overall booking this as commercial. The lease is key, and you would need to assign the lease as collateral (together with all other leases anyway).
Going the ways suggested, you'll be asking your investor to take a thrid mortgage and more than two mortgagees is usually not something a bank wants to get into, while there is nothing wrong having ten mortgages, if something goes wronge party the bank needs to go to with notices and and any take-out option not to mention a possible default on the third and having their mortgage either assumed or paid off.
Use an LLC, between you and your investor, in the operating agreement, you put the investor in a limited position, basically as a limited partner. Agree to indemnify him for any losses, Allow him to loan money from his capital account and the LLC pays him. He will own the % of the property to his contribution. Then you do a buy-out or option for you to buy his % of the LLC.
Ask the seller what he would like to carry, not so much how much he wants down, ask if he would carry 40 or 50 or 60%.
Go to the bank and have the LLC purchase with the investor. $600 of your payment will be assigned to the bank, it will be viewed too on the credit worthyness of the cell company, that should be strong and like gold if it is a US carrier.
The chance of you losing your cell tower lease will be slim and none, because the cost of removing it and acquiring a new site together with putting one up could be many times the value of your land, so they are not going anywhere. I'm surprised it's only a two year lease, usually much longer.
To sweeten the deal with your investor, offer the tax advantages to him, since if you are starting out, you probably don't need them say in the next 3 to 5 years, maybe 7. His option in selling his interest can be made today and keep the interest rate lower on that retun to him in the LLC, but the interest is offset with a premium.
The option can be made the day after closing too.
To the bank, the loan is to the LLC, so there is no debt service viewed for that investor money, just on the first and the second to the seller. It's a cleaner deal for the bank.
You need an idea of what the seller will carry back. Point out his tax advantages of gains being paid out as received, not all at closing and his interest income as you mentioned.
You can also do this as purcahse, note and sub-2 or as a contract for deed. But a straight sale is better IMO since the payoff is so low. If your seller has paid as agreed, contact his bank for financing, they liked the property or they wouldn'tt have the note.
I'm assuming this is in the country somewhat with a few acres. That might open the door for other types of financing (USDA?). If you do need assistance of any kind, you should be able to get free assistance from several here on BP. Good luck...
