I'm no expert but if rust and water are both potential issues, I would probably put both in series. First the 10 (with water control) to catch the "big pieces" and then the 2 micron to catch anything that gets through the first filter.
Have Fun!
Don
A setup like this would be just as good, if not better, than the pumps at the your local fill-up spot.
The only reason you haven't seen the rust in the bottom of the tanks at the stations is because you haven't looked.
After all, the majority of us are subjecting our CR engines to the mercy of the fuel the vendors are providing.
Other than the hpfp, does a TDI have any more of a need for clean, quality fuel than late-model CR truck and equipment engines? Probably not. What year are his trucks and tractors?
I've been driving a piece of equipment at work (late-model common rail diesel) that is fueled from a 30 micron gravity-flow tank and have yet to have any fuel-related issues.
If he buys good quality, clean fuel and filters as you suggested he'll most likely be doing his car a favor by avoiding the filling station.