I haven't found an answer to this specific question on BP or elsewhere online: if I die, and my SDIRA that holds investment property goes to my adult child, what are his/her options? 

Also are there any other options I can consider? 

Katie had several different options in her response that was quoted in your reply.

You will then have a few options with that rent money....you can put it into principal or you can start saving it to purchase yet another property.

There doesn't seem to be another option. 3) Do my spouse and I have to file two separate NPT returns for the rental income as we are joint federal filers?

Overall, there are a ton of really helpful suggestions in the replies here so I think you should definetley weigh some options in terms of if your goal is to keep the tenants or have the units be up to market rent. 

I am particularly interested in areas which I can either start earning right away as short term or long term rentals - Miami area or Bay area sounds like a good option for me.

Looking for long term financing options.

You would need to consult with a (local) lender to see what is available to you for that property, but I am assuming 20-30%.You would likely have 2 options with that property - 3 2 bedroom unit listings, or 1 6 bedroom listing. 

You may want to consider promo rates on an equity line as well and if they offer you the option to convert any part of the equity line into a fixed loan.