best heating boiler for MFR

[Joe Kato]

I have a 3 story 3 fam with separate utilities and the 2nd floor boiler need replacement in the near future.. Its stamped 1981, and there are burn marks on the outside of it.. The tenants are seeing high gas bills just to keep 1000 SQ FT of livings space at 62 deg. Looking at high efficiency gas boiler since there is $1500 rebate on 95% products, but I see these old boilers last for 30 years.. Are the new high efficiency boilers just as reliable? And should I get a combination boiler and domestic hot water combined, or keep separate for easier maintenance. I am taking a long view of this purchase, so don't want to think about it for 10+ years...

thanks in advance for any insight.