That is why we rent one. I consider rental as a type of insurance. My son's water heater is rented. It failed late one evening. They phoned the rental company and the company sent someone out the same evening to replace the water heater.

I know that it might not happen, but if and when it does, its a lot cheaper than getting a plumber out on an emergency basis to replace the water heater.

We have an oil fired hot water boiler as we heat by radiation. IIRC it has a small tank in it but once that is exhausted it heats by demand. During 6 months of the year or so where we do not heat the house at all we pretty much get by on 500l or so of oil. No idea how that compares with gas (which is unavailable here) or an electric tank. We certainly don't have room for a tank and a boiler!