Last house I did - purchased for 150k - put about 10k into it (And this was primarily DIY work - saved probably 15k doing DIY work).

You cannot charge one dime more nor a "pet fee" because service animals are NOT pets.If the applicant wants special features such as a wheel chair ramp, etc they MUST install that at THEIR expense and you can require them to bring the property back to its original configuration at the end of the lease at THEIR expense. 

Someone with a home in the mountains mentioned that we MUST install a furnace and cannot rely on the wood stove alone.

For sewer, our "grey water" (any water that goes down a sink or shower drain) is ran through a field line underground that we installed to distribute water underneath the year.

If you find a problem, deal with it immediately by making the tenant correct it within X days (and verify with another inspection) or remove them from the property.Most DIY landlords lose more money to simple mistakes than it would cost them to hire a professional property manager.

I have the funds to pay for the roof replacement, and I would pay for it ahead of the actual purchase, but hate loosing on the tax depreciation on installing a new roof since I'll be paying for something I don't own.

Each year at renewal we ask them what we can do to make their place better, maybe painting a room, installing a new fan,  .. anything up to $200,  so they tell us what they want, and it's fully tax deductible.  

Another thing to be aware of is the moving and installing of these homes.

If there isn't one present, has anyone seen any ROI benefits of the installation?

I was thinking of installing a new toilet but again, if they’re loading it up, I don’t know how much help that would be