What is your opinion on High Efficiency Combi heating units?

[Amir P.]

I am curious what everyone's experience is with the High Efficiency Combi (heat and hot water) wall-mounted heating units. I am currently going through the process of converting from oil to gas on a 3 family rental building and trying to figure out what is the best approach. 

I have one 2021 boiler, one 2006 boiler, and one very old boiler that needs to be replaced. All 3 water heaters are electric and toward the end of their usable life. 

scenario 1: Total $19,400

1 new gas boiler, $6K

2 gas burners to use on existing boilers, $6K

3 new water heaters, $3,600

chimney liner, repair, waterproofing etc, $3,800

 scenario 2: Total $17,625

3 new high efficiency wall mounted combi units $16,500

chimney work $1,125

I'd appreciate any insight regarding this process, the high efficiency units, maintenance, reliability, availability of parts, ease of use and repair or if there are any other alternatives I should be considering.