1.5hdi will pull 500 ltr no problem. My 1.6 pulls it like there's nothing in the back! I had a 1250 multijet that pulled 400ltr with no issues. My advice- buy the right van regardless of engine size. You're a window cleaner, driving between jobs, it's not a race! If the van is designed to carry 1000kg.... it will. Bigger the engine, bigger the costs.
I'm considering downsizing my next van as I've just discovered 1.4mm pencil jets and currently using approx 60% of water I used to with no ill effects.