I can buy investment RE but due to crazy bad DTI (far worse than 100 to 1, possibly approaching 500 to 1), I cannot get reasonable financing on an OO and not sure my co-sign will help on conventional financing.  

These loans allow buyers to purchase a property “AS IS, WHERE IS”.Renovation loans are available in 30, 20- and 15-year fixed rate terms and can be used to purchase owner occupied, second home and investment properties.By time the renovations are completed, the home needs to be in move-in, live-in condition and conceptually ready to be resold without repair issues.Other Important Items to Know about “Conventional” Renovation LoansMaximum – Minimum Purchase/Upgrade Amounts:Maximum: Limited to 75% of the “after improved” valueOccupancy: Primary, Second Homes, Investment PropertiesUnits: 1-4 unit propertiesRenovation Term:The renovation term for this program is a maximum of 180 days.The Borrower(s) is responsible for the work being completed within the escrow period.

Friendly reminder you can do a Conventional loan 95% LTV to purchase a home for them.

These markets often have strong population and job growth and strong rental demand.But the reality is that even in more affordable markets like Columbus, you'll need about $50k-60k household income just to qualify for a conventional investment property loan (this assumes good credit and reasonable debt).

Monthly expenses will include mortgage payments, property taxes, insurance, utilities (if applicable), maintenance, and reserves for repairs.You’ll need at least 3.5% for an FHA loan or 20% for a conventional loan, plus cash reserves.

Downtown Kansas City, Missouri has experienced $9+ billion in investment since 2001, including a new convention center and hotel, a state-of-the-art sports arena, a performing arts center, a new streetcar, and an eight-block restaurant and entertainment district (read more here).

I was shown about a handful of their holdings in the Houston, Memphis, and Little Rock markets, but the cash flow and CoC returns were less than $100 per month and 2-4% after factoring in additional expenses like 7% vacancy and 10% maintenance per month to go along with 20% down and conventional mortgage.

STR standard depreciation is 39 years @Andy Lanyi@Jonathan Cooper even if you take Bonus Depreciation as @Basit Siddiqi highlighted the remainder 40% will be prorated using the Half-year convention or Mid-quarter convention used in line with what you read in IRS Pub 946 

FHA or Conventional loan?

If you go with a conventional loan for the cash out refinance, seasoning periods are longer which draws out the process.