I think the ideal is probably the tankless natural gas models. 20 year warranty, no tank to rupture, instant and perpetual hot water. They run at $1,000 retail these days, haven't looked for online deals though to see price ranges there.
I assume a "combi boiler" is if you've got a hot water boiler situation for heating during winter ? It would make sense that your hot water boiler heating system could be dual-purposed to be run all year round and also provide hot water to faucets I guess. However, most modern homes in the US go with forced-air heating systems, not hot water or old skool steam boilers.
I actually fixed a hot water system for a friend years back, the copper piping had frozen and ruptured near his front door. I replaced the section of pipe and resoldered/sweated it with 2 couplings in-between, then filled the system up and fired up the burner to give it a go. It does have a very nice feel to it unlike forced-air, I'll say that, it doesn't dry out the air, etc. The forced-air standard is due to its dual-purposing for cooling/AC support in the summer, so you get both capabilities.