Like I can replace wood paneling with Sheetrock without a permit but I cannot put insulation into the walls without a permit.

Insulation - fiberglass, spray foam, etc5.

(And no, hiring a property manager will not insulate you from all the issues.)On the other end of the spectrum, A areas (e.g., East Side of Providence, parts of Newport, etc.) have very few issues and properties there really hold their value, but the investments usually have very meager returns because the properties are in such demand and prices are high.So it really comes down to, you'll need to do your own research and decide where on the risk/aggravation-reward/return spectrum you want to be.

The screened in porch slider was removed and the space was insulated, windows and walls put up to make it part of the living space.

The screened in porch slider was removed and the space was insulated, making it part of the living space.

They're kinda the sweet spot of "already insulated, have ductwork for heat if not AC, windows are not completely failing and rotting out the house" for me, and they're the most available MFHs in where I invest. 

is there an attic where you can check how much insulation is out there?

Also scheduled insulation work.- Tree removal, asphalt installation, roof replacement, windows replacement, deleading, pest control, interior design.

I have made a number of capital project improvements including insulating the entire building, renovating one unit, and installing two new water heaters.

We're going to tighten the squeaky floorboards, add some insulation below the sub floor then some LVP.