Ian,
Here's where I got mine, good info at this site:
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http://www.tanklesswater.com/ed_factors-to-consider.asp
I went with the Bosch 240 (now 250) which runs two devices at one time, but you'll still have a temp drop on output if they're both running. Look at the comparison chart, it will tell you how many gallons per minute through the unit at a specific temperature rise, that's the benchmark.
Mine's set to a specific output temp, 127 F I believe. New ones you can adjust with an electric thermostat. I could get one for mine as an attachment but never felt the need to adjust it.