Experiences replacing heating system

[Peter Fokas]

I have a 6 unit multi-family that has the heat separate (tenants pay heat). The 6 boilers are 25 years old and the 6 hot water heaters range in age (5 to 20 years). NJ is offering some financial incentives to update so contacted a contractor and got a quote.
Here are the details:
35k - to replace 6 hot water heaters and 6 boilers

5.5k to insulate attic

40k - total

6k in NJ rebates

so net 34k in total.

Since the tentants pay for heat this proposal does not seem feasible to go and be proactive about replacing heating system. Wanted to get other people's thoughts and see what others have done in the past. The heating system is working so I am under the opinion to leave as is until something breaks.