If the E says it'll talk to your XJ then it should be 95% of everything you could ask for. It'll leave a few features off that you might use if you were a serious ecomodder, but you sound more like a serious weekend wheeler. The E should suit you. It'll provide:
- water temp (you want that for grille blocks),
- avg and instant MPG,
- MAP,
- Intake Air Temp (very useful if you want to try a WAI for highway pseudo-lean burn cruising),
- TPS (also useful for WAI)
I think an E would do you pretty well.
For a softer but durable air dam, conveyor belt material if you have an easy source. If you don't have an easy source, find your nearest Rural King or Tractor Supply and pick up some baler belting. You may need to double it up to make it stiff enough to hold up against the wind of driving down the highway, but sturdy? Shoot yeah: it's designed to work inside a hay baler. It's tough enough. And if you have some left over you could extend sides down from your rockers, side skirts that, like the air dam, will give when you bump into something, but bounce back from it.
The hitch carrier can come right up to under the window and stop. All it has to do is fill as much of the wake behind your rig as you can make it, while tapering in from the sides (and up from the bottom) gradually enough to prevent turbulence from forming. Look up the
Aero Template to see what an "idealized" aerodynamic profile looks like. You don't have to apply the entire template to your entire car, improvements can be made just be tweaking here and there. It's what the automakers have been doing this whole time anyway.
Also read up on what
Aerohead has done, I think the guy is a literal rocket scientist. Even if he isn't, he's done what looks like Basjoos-level aero work on an otherwise stock T-100.