STRs are a great way to make money, that's for sure...but it does not come without risk.

I'd focus on deciding what you want to do (flip-high risk, but potentially high return...or large loss; short term rental-more work, but higher return than long term rental; long term rental-less work than the other two, but lower return).Once you've decided which you want to do, then focus on getting the best house to meet your goal. 

It's also important to know when you face the risk of a lawsuit.

Instead of putting your whole venture at risk through a single partner, I suggest you build a team of employees and develop solid succession planning.

I may not have explained myself clearly, so I apologize if there was any confusion.You are absolutely right that to carry out tangible commercial activities in Spain, such as real estate projects, it is necessary to establish an SL (Sociedad Limitada).

Is Japan a good place to carry out that strategy?

Diversify if you can with smaller amounts so risk is distributed. 

And as I explained earlier, your investment is protected by real property, so your risk is extremely low. 

Mortgage rates are based on what lenders consider risk factors such as loan to value, credit, etc. 

This is not something I would take a risk on, but maybe others would.