Even if you did entire build yourself, I think you are looking at 2.5 times your estimate and a lot of work and time.I suggest you go to some meet ups with an emphasis on understanding the value added by ADUs.  

I suggest what Chris posted. 

My research also suggests Roku may be an easy way to handle it.  

Research high-rental areas, plan for repairs, and consider simple upgrades.

I would love to hear any advice or suggestions on how to proceed...We just closed on a place in Puerto Vallarta.

I suggest you write up a contract with your seller and take it and your GFD to a local title agent. 

If I sell the house, I would like to use the money for real estate investments and would love to hear everyone's suggestions.  

Nathan's search method usually works, but I use what Google suggests: '<search item> site:www.biggerpockets.com' (without the single quotes on the Google search bar.) 

Because it will be very hard for you to know if he is selling you a deal in a good neighborhood, in good condition, and then placing good tenants.As others have suggested, see if there is a syndication deal that fits your investment goals.

The answer is very simple: if it was that easy, everyone would do it.