I have a vacant lot (almost .5 acres) overlooking a lake in Payson, AZ that my Dad gifted me over 12 years ago.

I would expect the cost to be about $800 to $1,200.On the other hand, perhaps you would consider gifting it to your son incrementally.

You should look at any limitations on gift taxes though, and she may actually want to structure it around any of this.

Read and familiarize yourself with these:Residential Rental PropertyBusiness ExpensesBusiness Use of your HomeTravel, Gift, and Car ExpensesSchedule E InstructionsPrimarily focus on "Residential Rental Property", "Business Expenses" and "Schedule E Instructions".

I’m interested in giving my son a house downpayment gift via setting up a trust for him to use.

Assuming you are married gifting each child 36k a year is a wonderful start.   

Unless you can purchase the first home as primary and show you will be buying the next FHA home closer to work, or out of state.There is also something called a "Gift of Equity" where you do not have to assume the loan you can use gifted equity for a down payment and closing costs.

On renewal I would do market rent and whatever you want to give for disruption and noise give it as a gift card or some similar not a reduced rent. 

Grateful for friends in the BP community who've gifted me "The Book on Managing Rental Properties" by Brandon and Heather Turner.With so much to learn in such a short period, I'm eager to hear any advice this community has to offer.

Or see if someone is willing to give you a "gift" for the downpayment. 3.