If I'm down there and see a property I love, I would probably still buy it, with a regular mortgage.I do have to figure out where I want to live mostly, but some of that will depend on what I can do with the money and how confident I feel about longer term stability.Renting out a Spanish property while I'm in Canada seems somewhat easy, as it's pretty common there to rent out fully furnished places, and tourism rentals would be an option too, to help pay for it.Canada on the other hand, I'd have to put everything in storage or sell it off every time I want to be away and rent the house...

I currently have 5 rental properties that are all single family residences and look forward to expanding my portfolio to contain more income properties like storage sheds, RV parks and commercial properties. 

Without having any experience with this, your best bet would be to talk to a Civil Engineer, if after talking to Planning it seems feasible. 

Is there a good book that describes how to invest in self-storage?

This would be my first endeavor in RE and it may be a serious stretch for a newbie but I'm willing to try if it makes sense and is feasible. 

The tank is not a storage device, though. 

This would be my first endeavor in RE and it may be a serious stretch for a newbie but I'm willing to try if it makes sense and is feasible.

What costs can I assume just for feasibility of project?

The property is 1.5 acres with the 5 unit apartment and a 4,400 sq ft building that could be storage facility. 

I don't think it's feasible to do the firewood thing in this case, most of the bigger stuff was pine, and any hardwoods were mostly small brush.