I think they would have the original plumbing and likely the electrical also.

Newer plumbing, electric, brand new roofs, vinyl siding, all 15 are carbon copies.

So, the total project cost is 43,675.00 USD and I can get a 4-year personal loan of 44,000.00 USD with 4% interest.The average monthly income in the last 3 years for investors in this eco-resort has been 600 USD.The totality of monthly expenses paid to the eco-resort for the management, water, electricity, gas, etc. is 187 USD.

You can also sister new joists to the old ones, that was harder for me as I had a ton of electrical and plumbing running along and through the joists. 

Is the electrical cloth-insulated?

I'd guess 1600 per unit,, washer 400, dryer 350,, (oh is it electric ???

You won't have any trouble renting it, even with window units, but you won't be able to keep a tenant because they won't be able to afford the electric bill.

Many contractors are used to doing full gut rehab but it's rarely worth the time, money or energy when you can create a similar looking result with some new flooring, tile, paint, fixtures, trim, or decorations.

Check electrical, HVAC, roof, have the main lines scoped.

Both were gas units.So my own experience with single residence tankless heaters is mixed but I am still a fan and have two units that I will convert to tankless when the current water heaters go (because there is only electricity - no gas option).Good luck