My biggest REI shortcoming is that I am not an expert on construction, carpentry etc and I know I have been taken advantage of a few times in terms of inflated repair costs.My biggest lesson from this is to collect 2-3 quotes for large jobs before awarding the project to any contractor.

If the loan is valued by the rents how do I calculate for the arv.Example6 Unit Property Cost - 850,000CURRENT RENT - 5,300ARV - ?

My CPA is saying I have to pro rate those expenses for the number of nights that it was actually rented out, but I can't find anything online that confirms that.

One way around the 20% down is to buy a house you will live in and rent out the other part (think duplex, triplex, etc).  

some wholesale, but mostly buy and rent to own.

Mexico City, Tulum, Merida and Puerto Vallarta are in my list however I do not know which one would be the best to: Buy and flip, buy and rent or buy and Arnb? 

Especially if the condo can be a STR, an ocean view makes it more valuable...not to be a "butt head", but people can buy/rent and look at a parking lot anywhere for much less.If you think you've got a good agent, take their advice. 

This tenant has lived in the home for 10 years, and pays a VERY low amount in their monthly rent. 

Not everyone starting off gets hit with this, and if you can buy a portfolio it can "de-risk" the issue to some extent, but if you have one property and experience an 8-9 month fight to collect rent prior to eviction with a large repair bill after that it can be very painful.ALWAYS use a licensed PM in Detroit. 

You could try house hacking, living in a multi-unit property and renting out the other units to cover expenses, or look into areas outside the core like Aurora, Lakewood, or Westminster for better cash flow opportunities.