I’m building a similar size rig and I went with an eccotemp L10 and a shurflo 4088 (3 gpm) both off of amazon dot com after a lot of searching around.  The L10 was basically the biggest capacity I could find for just over $200.  The tankless units get bigger and hotter but the cost goes up quickly.  Eccotemps, or the L10 at least have a sensor that shuts the unit down if it detects a temperature over 150F but it can be disabled easily.
 
Also, I found a deal on a solar water heater panel that I’m going to use to pre-heat water.  I’ll store the pre-heated water in a hot water expansion tank just upstream of the L10.  I’ll also use an expansion tank for the cold water side.  These provide a reserve of pressure in the system so the pump will cycle less often and if you need to crank out 6 or 7 gallons one minute, a 3 gpm pump can catch up later.