Mine was installed 17 years ago, in my condo, with existing 110v electric and same gas service as tank heater used. Recouped the $5,500 cost in under 6 years.
I had a small tankless unit installed in my Nursery/Garden Center Main Building Office, when I built the 20ac facility, in 1990. Cost was about $1,000 during construction.
No need for anything unusual — unless you’re planning to supply a whole condo or commercial complex, and they get better-performing units, every 5 year cycle, or so.
A very small tankless unit (suitable for a 1 bedroom, 1 one bath condo) might be OK with the gas service supplied to a regular tank heater, which is around 30K BTU.
But a small tankless unit won’t be able to provide a high temperature rise at high GPM (this might be OK in places in the country where the incoming water temp is higher).
In other words, the colder the incoming water, the higher the gallons per minute being drawn—the colder the water that comes out of it.
So if you want to be able to draw 4GPM with a 40F incoming water temp (common here) and an 80F temp rise (120F out), you aren’t going to be doing it with a gas supply line capable of 30K BTU or a 30 amp 240V circuit. Try more like 150K BTU or a 120 amp 240V circuit. (usually spread across 3 40 amp circuits from what I’ve seen).
Those are expensive to install.
Put another way, if you’re able to make your existing tank water heat run out of hot water by running your shower long enough....
...you aren’t going to be able to use a tankless water heater without a larger gas supply line or a larger electric circuit, as applicable.
Tankless water heaters do NOT have some magical ability to heat water any faster than one with a tank, given the same gas or electric input.