If I buy a new property as a primary residence and move into that new house, will I still have to pay a down payment for an investment property if I plan to rent out the house I currently have after I move into the new house?

If they will take interest only for the life of the loan, that would be preferable to amortizing as you can likely do better than 5% interest with the principal money you wouldn't be paying.

I don't want to pay for a separate subscription for each of my LLCs.

If you’re talking about paying over asking or having a larger downpayment: Those aren’t  expenses.

The x-tenant managed to move again and get a new car just this spring, so no motivation to pay us since it's apparently not crimping their style.

They have 529s to use to pay their living expenses and school tuition. 

So I wouldn't trust a bid from a Contractor unless you pay them, the other guys will just mail it in.

Devin,Just a warning about checkbook IRAs: Self Dealing Activities will get your entire IRA disallowed and you will have to pay the tax on all your money.Self Dealing Activities are frighteningly easy to do.Do a Copilot search for "Problems with checkbook IRA plans."

What is the best legal structure to assume his existing land contract while either concurrently paying him or writing up a 2nd land contract to the seller past the 4 years on the current land contract?

I'm guessing the management company would collect all rents, pay expenses etc and the individual LLC would just own the real estate.