An issue that I am currently working on right now is keeping track of the heat in my vacant rentals (25+ units at any given time).

This equated to a 4% ROI for my 1st year.Lowering some of the expenses like snow removal, landscaping, electricity bills and heating bills equated to a 12,000 per year cash flow or a 8% ROI.If I raised rents 25$ per unit my ROI would go to 11-12%.What is the highest price you would pay for a property like this?

Are there a lot of places heated with oil tanks still?

Truila has a crime heat map you might start with that.. high crime is not good as a landlord.. low crime is better.. that would give U a macro look at the market as a whole..  

You typically have to provide heating, electrical, and running water.

When you do rent it, I would set up the leases so they all end in the warmer months ( this is what we do) that way when you do have a vacancy you are more likely to rent it sooner and no cost to heat when warm out.And as @Cassi Justiz mention pictures are very important, bright, clean and staged always works well.

Breakdown, 2 units are on raised foundations allowing for central heating/air but part of the main house (3/2) is a slab foundation along with the studio. 

You need to see when the major systems were updated, especially complete electrical which could have post and tube wiring or Al or silk wrapped all of which would be beyond their life, plumbing for the old cast iron pipes that split and if there is a heating / cooling package. 

We've taken pretty much all approaches, depending on conditions (excavation, hand-digging, heating, etc).

It seemed that there was some discussion of fit and finish of the area, but the key component was the permanent heat source.