Since owning the home we have a installed a new roof, and downstairs heating/cooling system, as well as repainted, fixed cracks.  

Particularly as back-yard dwellings, these will likely inspire some heated discussions at city council meetings!

If you want to focus on Buy-and-Hold and your goal is to build cash flow......I suggest looking at markets where Turnkey operations are heating up.You want low purchase prices and relatively high rental income (another words, high rent to purchase price ratios).  

I like your ideas of turning up the heat a little (especially if the tenant is paying for the utilities) and installing a low flow shower head. 

However there's only 1 water heater and 1 furnace to heat the water and radiators (its an older rental).

Heated floors, USB outlets, special features in the kitchen?

Easy to get caught up in the heat of the bidding and overpay.  

If it's worth $790 for the piece of mind that you won't have to worry about shelling out thousands of dollars for a new A/C unit or heating system then you should do it.Best of luck!

The new construction buildings have higher ceilings, have more isolation in the walls, and all the common elements are brand new: brick work, roof, windows, heating system, etc. 

Same price as our multis, but new instead of a century old and a fraction the sinks, toilets, heating systems etc.