Old Utica Oil Boiler

Making domestic hot water with a tankless coil is probably the most wasteful, inefficient way using the oldest of technologies.
Production with a clean coil is limited to about 3-4 gpm. Then as it gets colder out, cold water supply temperature is lower (colder) and hot water production is lessened. Add in limed up coil and/or failing mixing valve, and you don't get much domestic hot water out of the boiler.
Now you turned up the boiler temperature wasting more energy (oil).

You're better off with an indirect on it's own zone, whether or not priority is needed. Better, but can't call any option 'cheaper'. You're boiler may be fine and adding an indirect would work very well. Plus, when you do need a new boiler, you'll already have the indirect storage tank.

And if you need a new boiler, the best, most efficient one on the market is an Energy Kinetics, with or without a storage tank based on your needs or the model. The EK's are easily converted to natural gas should you eventually get it. But, to run a gas line usually costs thousands of dollars as they don't subsidize them anymore, and you'll never recover that costs. You also may be in an area that isn't allowing new hook ups to the gas grid.
Second best option would be any triple pass boiler with an indirect.

Now the second issue might be too far a run from the boiler to the point of use. There are a few methods to overcome that, depending on your situation.

I agree with everything that Steve posted 100%. Going from an old boiler with a tankless coil to an Energy Kinetics boiler your oil savings will be dramatic. This is your best option.
If boiler replacement is not a possibility due to cost and the existing boiler is in decent shape than adding an indirect tank would be the next best option. During the year when you are not heating the home your burner will only fire a few times a day to keep tank 120-140 degrees, rather than keeping the boiler 150-160 degrees 24/7, 365 days a year. A mixing valve on the tank will help efficiency even more.
The least expensive option is to install an electric water heater. This will require some extra space for another 240 volt breaker in the panel, as well some wiring, but in my case it was expensive on the monthly power bill, and the tanks don't last very long unless you buy an expensive one. I usually don't recommend tankless water heaters of any type.