Based on your fact pattern (was a personal residence first and then a rental, not vice versa) I think you might be looking better.

Personally I don't really care for the upgrades.

Keep the main podcast David and Rob!!  

There are no laws, rules or regulations preventing any foreigner, American or otherwise, to buy property in CDMX or Mexico for that matter.If, by having success, you mean getting a profitable investment, I'm sure many American and other investors got that over the last few years (I personally haven't since I've focused my efforts on Mexico's beach markets, which are far better in my opinion.

Or what if you personally found home you want to buy without a real estate agent...is it recommend to engage an agent?

I've seen an overwhelming amount of utterly freakish craziness up close and personal, and I can confidently bear witness that white people always end up being the freakiest freaks of all.

What do most do in terms of keeping rental business out of personal finances?

Flipping homes is NOT something a person just starts at, that's insane. 

Personally, I've been focusing on understanding local market trends and tailoring my strategies accordingly.

They're offering 3.99% interest rates (covering 6% of the home cost to buy down the rate), $10k in bonus money, plus a fridge, washer, and dryer.Because they can't stop building, they have to offer extremely good incentives to keep the product moving in this buyer's market.