Due to item 1, adding an ADU can require years to start achieving any return (once the accumulated cash flow recovers the initial negative position).4) Adding an ADU is a slow process.

Again, the BiggerPockets store has some books on this topic, or you can learn about it by watching podcasts, reading blogs, and interacting on the forum.

If so, are you concerned about Spain's tax system on worldwide items?

-When a tenant moved out, the management company refunded their security deposit but failed to account for damage, such as holes in walls and outlets ripped off the walls, leaving me with repair bills and also tenants moving in without the repairs being done.Stuff like this that is consistent and when I ask questions they are defensive, when I ask for more quotes for high ticket items they are defensive. 

I've been pondering the same question lately.For items like HVAC filters, do you remind tenants when it's the time to change it, or just expect that they will do it?

That's it, but a) print out a page for each property you think is comparable, b) make the list of repairs long and inclusive of everything you've done. c) Put larger items first like kitchen remodel......and don't just say 'kitchen remodel', list things like 'custom backsplash tile/Bosch appliances/recessed can lighting/pot filler/etc. d) Cite the price/value of the work done, and don't be afraid to be generous.

If, however, there are any gaps at all in any of the above items, even a gap so small a mouse can squeeze through, that tenant will exploit it and your life just became a 7th Level of Dante's Inferno.....:)

With that being said, in a worst case scenario, you can always hire a property management company to handle the day to day items if it comes to that. 

Due to item 1, adding an ADU can require years to start achieving any return (once the accumulated cash flow recovers the initial negative position).4) Adding an ADU is a slow process.

The BiggerPockets store has books that will teach you how to analyze properties.