My buddy owns a business and rented a 25,000 sq/ft building (not pole barn) for $1000/month.

We decided to take profits from renovating it to fuel our investments.

Snow removal, heating fuel, etc. not huge expenses, but something to think about.

This is just Fuel to my drive to get it right THANK YOU

Louvers or louvered doors, bought new, will say how many square inches of open area they provide.It is best to get the manual for the exact heater you have from the manufacturer, but if you can't, look at a current similar water heater at the hardware store (size in gallons, fuel, efficiency level) and get the manual for that one.I used to live in a house where the water heater was in a closet behind the laundry room. 

Don't just count the actual time doing a task, but include the hours and expenses (fuel, etc.) in getting the materials (if you picked them up yourself) and more importantly, the overall timeline (from first touch to last) that the DIY tasks took (e.g., you might be installing flooring in one room, while your wife is painting in another, but neither of you could then be working on the landscaping at that time, whereas contractors should be working on these things simultaneously).  

Separated the heat so the tenants paid their own heat, hot water and range fuel.

Drain pumps on both of those are plastic (usually) and will crack when the water in them freezes.Before draining the hot water heater and heating system, the fuel source to them is shut-off.The water heater will be drained.Boilers and radiators will be drained.Not sure about sump pumps - you would want them to pump out water, but if things freeze ...Not a pool guy, so I have no idea about those.After winterizing, there will be signs posted (taped on) all over the place identifying that the place has been winterized and who the contractor was that performed that task and their contact info.

If the tubes have been removed (but tank maybe is still there - who knows), you can sometimes tell by looking how the fuel lines travel within the house - if they go towards an outside wall, and there was no above ground tank ...I think all of the items Vikram identified in his original post are possible issues.

The required guidelines seem to change for the worse every day.FYI, If your neighborhood is fueled by bank and REO comps(which is easy for anyone to figure out), those will be the comps.