An indirect is pretty reliable and efficient with a Mod-con boiler and outside reset control. The better indirect tanks only lose 1/4-1/2 degree per hour, which tells you something about their insulation. You'd want to put a tempering valve after the feed for the DW, but before it goes to the sinks to keep them from scalding themselves, but most systems run the tank at 140 or so. A fairly small tank with nearly any boiler should handle that small peak load of potable hot water. The better ones are all SS, and essentially have no maintenance and a very long life. Look at the first hour ratings on the indirects, probably even a 30-gallon one would meet the 50gallons per hour needs, or even smaller. The mod-con with a small quality tank might only need to run once a day in the summer to maintain the tank set point. You didn't say where you are located, as incoming water temp will affect the size tank you'd need as well as the boiler.