If you're using a presure washer on the tank, if you get the right one, you won't need a pump as well. Quite a few pressure washers have the ability to suck the water out the tanks themselves.
Getting Honda engines would be your best bet, you can plumb them and mount them fairly easy and the will be your most reliable. I have a honda generator and power waser mounted to the back of my trailer with the 100 gallon water tank in the middle. You can get all the hoses and valves from Home Depot.