Well, it's not suton am guna set up but if I was ......
There's a 210 litre tank on eBay this would be ideal ( it's only for adding the chems a 25l barrel will do ) if u have the space next to wfp tank, rig a shurflo pump upto it and run it to ur wfp hose reel, out the brush and onto the surface, ur wfp tank is running water Into ur smaller tank/barrel to fill as u emptying it, ok dilution rates will be a bit variable but u there.
Once the area is all treated the second tank is being cleaned by the water from the first tank running through it and then u can just switch back to the wfp tank to do a rinse/clean down.
I have used my wfp tank and just disconnected everything after the pump and just gone direct to the hose reel from pump, once finished just a good cleaning of the tank is required.
Little bit different if u planning on going the the honda engine route say.