Need to replace an old oil boiler in my 4-unit multi/primary residence. Currently has an ancient oil boiler which is at least 50 years old, held together by duct tape and unmentionables. Barely got us through last winter... so it's time. We have a few options and I'm looking for feedback.

Also, for context, right now we include heat in our tenants rents, but they pay their own electric / hot water.

Options:

1. Replace with a more efficient modern oil boiler. Quotes we've received indicate we'd also have to replace the oil tank which is a bummer. This is probably the most straightforward option. We also have four aging water heaters all past their dates, which we've been encouraged to replace with a single indirect water heater (so we'd start paying tenant hw bills but could up the rents next cycle to accommodate).

2. Natural gas runs to our street and Unitil will run the line to the house free of charge. They could install a single natural gas meter, but they have to drill through our retaining wall to do it, which is a bit of a concern but apparently nothing they haven't dealt with before. Cost for the boiler and labor is roughly the same as the oil setup. Same situation with water heater.

3. Have Unitil install 4 meters and install 4 new combi wall-hung viessmann (or similar) high efficiency units. Turns out our heating zones are already separated so this isn't as expensive as it could be but the total bill still comes out at about 2x the cost of just doing the single system.

From your own experiences, is it worth it to spend the extra on the 4 separate systems? (and then later adjust leases so tenants pay their own utilities) It's a significant extra cost, though I do like the idea of the combi units in principle. Good/bad experiences?

Is there any reason at all to stick with oil given these scenarios? The only one I can se2e is our general anxiety about Unitil boring through our retaining wall heh.

Also, any opinions on the indirect water heater systems I mentioned should we stick with a single system?

This is all new to us so we're doing a lot of research but we'd really value any feedback from our more experienced neighbors. Thank you!