We hired contractors to construct the basement unit and turned 300 square feet of unusable space to a $1,000/month of rental income.

U want your parents to talk to their CPA FIRST.. if they go the development route they are now ( depending on the amount of units) creating inventory.. which will be taxed at ordinary income rates ( very HIGH)if they just sell they can 1031  and move on to something else.. many times when you run the math consider the risks of a partnership.. the sale of the land is far better than thinking your going to make some profit.. 

I cross-list my MTR units on Furnished Finder and Airbnb.

So, the following comes from the perspective of a small-portfolio landlord who is a DIYer who lives close to his units and doesn't want to pay for a Registered Agent and PO BOX.

Electric heaters for apartment units.

The suggestions of getting a 2-4 unit multifamily are great too. 

Total ARV rent for 3 units is $2500?

You can even house-hack and rent out the other units with STR.

Then re-calculate the estimated rent for your unit based on the comps you selected from their reports.

The MLS is not the best place to cruise for a bargain but I was looking at some 2-4 unit properties and the prices are absolutely bonkers.