We feel that by having an smaller home we will be more connected and will have less unnecessary stuff.

The ROI on solar is more complicated than a one line answer on a forum post. 

Sounds overly complicated with little-to-no benefit.

Assuming that is close to correct, I think the owner will likely find this complicated and make it a hard sell.  

Complicating the matters are 3 tenants, non of them paying rent. 

It may play out well for you, but the details are tricky, and the adjustment procedure is complicated.

you are not alone nor dumb, i read an American study that 40% of home flippers lose or only break even on their projects. please talk to an attorney with bankruptcy and real estate experience before filing as could have long term complications.

Source: https://es.wikipedia.org/wiki/Demograf%C3%ADa_de_Espa%C3%B1Trust me when I say Spain is in a very complicated situation.

In that case just defer doing unnecessary work to the unit. 

Things get more complicated if you have a legitimate reason to believe the person is lying to you.