What kind of furnace (and AC??) do I need?

[Ashly B.]

Well now I'm diving in to the really noob questions. Our other investment property was 100% flipped 2 years before we purchased it so I'm questioning what areas are worth spending money and what aren't. First up is heating and cooling.

Quick background:

Buying a 4 plex in a transitional neighborhood. Current tenants are all lower income. Units are all currently rented but as they turn over, we'd like to update them and work toward appealing to the young professional crowd. Posted before about one (not in contract) tenant who is not taking care of the place where we found roaches during the inspection and they have 2 dogs that are peeing everywhere. We plan to give them notice to leave once the sale is final and use the opportunity to make that the first to get updated. 

Questions:

3 of the 4 furnaces are from 1984. Currently there is no ac. 

1) Do you suppose there would be a large enough discount to make it worth going ahead and replacing all 3 units at once or should we wait and do them as we upgrade each unit. If its relevant I anticipate it taking 2-3 years before we work through all 4 units. 

2) Where do you source them? Do you just go buy from a box store or are there better options for investors? 

3) Since the units all have their own furnace, I presume I can get a smaller size? Each apartment is about 800 sq ft. 

4) Wondering if its worthwhile to do heat and ac together? Currently tenants would have to provide their own window ac. Not sure that offering central air has THAT big of an impact on rent or marketability. FWIW we do have hot summers and most people use their air from may to september.