If I were to do STR, I would consider getting a new hot tub or getting the old one in working order, but I am concerned of the risk it adds to the property from all the moisture since it's indoors. 

One other tip is to be sure all of your offers are contingent on a moisture termite inspection, even if they are as/is. 

There is moisture getting into that wall somehow. 

Expensive appliances, they are, when compared to the standard USA dryers.But if these are the typical dryer found here in the USA, then the moisture is expelled through the exhaust vent; failure to properly vent these to the outside would lead to high moisture / humidity levels in the air within the unit - one of the things that could lead to mold developing.

When rain (or moisture) come, they expand to cover the gaps that are there naturally from installation.

OK, I have some major updates regarding my investment with MI.I had an inspector go out and take a look at it during construction and he found MAJOR problems ranging from possible asbestos insulation being airborne, lead-based paint cracking, cracks in the foundation and crawl space, old knob and tube wiring, moisture and mold issues, roof shingles replacement, and loads of other issues.Rather than talking about them, I made a playlist of the inspection here: https://www.youtube.com/watch?

That attracts moisture and vermin.

For example, there is no insulation under the floor or around pipes, part of the roof needs repair and half of it appears to have 3 layers of roofing material, single pane windows presenting a moisture problem, some failing siding, etc. 

Lack of ventilation allows for the moisture to build.

But it's not cheap, and I don't think you really need it unless you're having a significant moisture problem.