The easist thing to do is to just hook it up in series: The cold feed goes only to the cold side of the tankless coil, the hot feed of the tankless coil goes directly to the cold feed of the water heater, leave the water heater's aquastat settings alone. There needs to be a tempering valve on the output side of the water heater to keep from potentially feeding 150F water to the hot water distribution plumbing.
Then, set the low-limit of the boiler to anything you like (140F min for most oil boiler)- you don't really need it to be very high for good hot water performance anymore, so set it to what's needed for the heating system, which could easily be 150F or less. Most of the heating season the boiler will be supplying the bulk of the hot water to the tank. During the summer you can turn the boiler off to save on standby losses and let the electric tank cover the water heating load. Any time there is a hot water draw, it will pull heat from the boiler into the water heater, and the only power use will be "finishing" a few degrees and covering the (fairly low) standby loss.
If you wanted the boiler to do 100% of the water heating as a kludged up indirect HW heater it would involve more complex plumbing and a pump, but in most cases the operation cost is cheaper if the hot water is only heated by the boiler during the heating season, since the duty cycle for water heating-only is so low during the summer that standby losses add up to something between 1/2-3/4 of the oil use.
If you want to go the kludged-up indirect route, use the contacts that run the upper element of the water heater to run the pump for the loop, and disconnect the bottom element. Tee-in at the drain valve point of the hot water heater for feeding water back to the tankless coil, and tee the hot water output of the water heater for the coil output, leaving the cold water feed to the electric tank where it is. The pump has to be bronze, suitable for potable water and higher temperatures, not a low-duty-cycle type hot water recirculation system pump, and not a heating system circulator.
