@Jim Sestito I just did this up in NH, total no-brainer, but depends on how old your existing oil boiler and DHW tank/heater is. If they are newer, it will take a long time to recapture the investment.
You can get combi units that do both heat and DHW. My rinnai combi was about $2500, but there are some cheaper ones. There are Energy Star incentives for rebates, and i believe you can deduct the cost on your taxes for energy efficiency (may have changed for 2018, not sure).
You may have to upgrade your gas meters (free from the gas company), mine were rated at 220,000 btu and the tankless pulled 260,000 at high fire, so it constantly shut down due to lack of gas until they were replaced with 440,000 btu meters. Check to see what your meters are rated for and get the gas company out there to update them ASAP, as someone mentioned above, it can take a couple months.
Also, you can frame up and plywood the area where the oil tank was and offer your tenants on-site storage for an extra $50/mo.