In another life, I used to repair/maintain hot pressure washers for a big rental company.
You won't need auxiliary pump if you if your heating barrel is elevated above the pump. Gravity will do the work. Believe it or not, pressure washers don't consume high volumes of water. One 55 gallon barrel should be enough for most jobs.
As far as the hot water killing your pump, it depends on what style of pump it is. Another consideration is the wand and the hose. Burnt hands and burst lines are no fun.
Nothing cleans a freshly machined block like a hot pressure washer. When you are done the block is dry already. You can watch the flash rust form within seconds.
