Replace heating systems in one unit or all four?

[Nicholas Lehman]

I purchased a fourplex in the Anchorage Alaska area earlier this year. Each unit has its own forced air heater and 48 gallon water heater. The furnaces are 20-36 years old and the water heaters are 10-16 years old. Presently the furnace in the unit i am currently living in is not working and the home warranty company is trying to say that the repair is not covered under their exclusions so i am considering replacing the whole furnace on my own. Since the hot water heater is packed in this little mechanical closet with the furnace i am wondering if i should also replace the hot water heater. Also the heating company has offered to do a small discount if i do more than one unit. Should i replace the furnace and hot water in my unit? Should I also consider replacing the furnace and hot water heater in another unit at the same time and maybe next year or so replace the heating systems in the other two units? Or do you recommend only fixing or replacing things as they break? Thanks for your suggestions!