Also keep reading BO posts.  

Then sell the property and start doing fix and flip or fix and rent. ( I prefer to sell this property as it 100km away from my student rentals. ) Not sure if I should renovate the lower unit to sell.

You really need to talk to a tax advisor in this situation.

Although multi-family properties is a term used more in the US and converted itself into a proper investment strategy, this is also possible in Spain (or anywhere in the world).

This is not going to be changed.  

This also leads owners to ASSUME simpler is better when it comes to management contracts.

I’m not sure how many of the previous responders read your post because they didn’t even ask obvious question number ONE….1) How much of a capital gain will you have?

What I read was you pay taxes on it not cap gains.  

Besides reading and listening to podcasts, my wife and I attend a local real estate investor meetup that has been extremely beneficial. 

This is a great place to learn and to connect with people so take advantage of all the great resources.