I've personally never used opportunity cost as an expense, considering it is more or less intangible.

I bought a house that cost a little more than the LoC amount, used my savings to get it rent ready and rented it to a tenant.4) Every property is different. 

HVAC guy sold me the equipment at cost as long as I paid him for it up front.

Separate loans will cost a lot more, since you'll have four full sets of closing costs.

With that being said, Spain is a great market to buy into right now, particularly along the Costa del Sol - a glut of supply and grossly-depressed prices make for some excellent long-term appreciation gain, as well as an established and well-entrenched demand of STR in the summer. 

When you find a good property management company they will care, it may cost a little more, but in the long run they will get you higher rents and lower occupancy.

Conversely, contractors are physically working on homes all day and know how to fix things and how much they cost. A

You mentioned buying with cash, so maybe you could even pay more in cash and they would allow a lower rate of coverage and less cost. "A

My point here is that if you are going to look at every cost as a negative, then you might as well look at the interest you're paying on the loan as well and try to payoff the loan ASAP, before moving on to the next property.There are costs that are true Expenses...tax, routine maintenance, utilities, etc. 

So, it may cost a homeowner $50k to do a renovation, whereas the rehabber can do it in $30k.