I'm just going to add my 2c here and ask if changing the car over to one with air suspension and stock tow bar might also be an option? A lot of time and expense to go to with new suspension, lift kits, etc etc. Is there something special about this car? Air suspension will fix your problems, and when you upgrade the toy hauler (as you suggest) you won't run into a whole new set of issues. You'll also gain the extra HP as air suspension usually accompanies the 180kw engines. Anyhow, at the very least I'd try a WDH as they will help, but I'm not certain they'll get rid of that much sag.