Advice on heating/cooling budget for duplex renovation

[Brandon Cline]

I'm looking to purchase a duplex that basically needs a full renovation, but I want to see what some of you have to say as the best way to go about tackling the heating/cooling situation. Currently each unit has a swamp cooler unit on the roof with one duct to the inside, but they're  pretty much toast.  Each unit has a wall heater. Each unit is a 2 bed 1 bath and 900 sqft. And the entire roof needs to be replaced.  For those of you who have some experience with this, where do you get the best return when comparing going all out and putting a new furnace, ducting, and forced air to leaving the wall heater and replacing the swamp coolers? Since I would be doing a full reno, is it just better to get the ducting ran, the furnace installed in crawl space, eliminate the wall heater, and bite the bullet now? Kind of nice to punt those major cap ex down the road, but it is a rental and I don't want to over improve the units if that's not smart either.  Another option I looked at was maybe installing one of the ductless mini coolers, but wasn't sure how those perform and if it's just one more thing to be sticking out of the wall and risk being damaged.  One thing to note is tenants pay for gas and electric, owner pays for water.

I'm an AC rookie because where I live we don't need it, so I'd appreciate any feedback from the investors out there that have any advice. Thanks!