If the property you're evaluating has "good bones" i.e. the structure is sound, then being an old home is only a deal-breaker if the cost of updating the plumbing, electrical & mechanical is cost-prohibitive given your Return on Investment goals.

I bought an apartment in Cancun in 2012 and had to sell it because the management prohibited short term rentals...

If I take on large debt will that prohibit me from taking on new debt when I acquire other properties?  

By Colorado law, there is no zoning ordinance that can prohibit the use of a factory-built, U.B.C.

For the most part, I've been talking to HOAs directly and carefully reviewing bylaws/CC&Rs to see whether they can legally prohibit my clients from STR.

I did put one in a 3rd floor unit which has been successful but its classified as efficiency apartment-any larger of a footprint I think the cost would've been prohibitive.

I don't have any approximate price/sf to give you but of the options I know, the cheapest would be putting another layer over the existing but that depends on how many layers you already have on the roof (local code may prohibit more than two layers).  

There doesn't appear to be a personal benefit, but I always recommend checking with a qualified professional - CPA or Attorney - to make sure you are not running afoul of any prohibited transaction rules.

Earthquake insurance is separate and prohibitively expensive and almost no one has it.

.- We prohibit open grease cooking, meaning no fish fry or fried chicken on the stove top.