Be careful with dually kits. "If" you actually need dual real wheels, you should get a dually axle. If you just want it for the looks, go for it. The kits just add in a spacer with long studs to fit the rims. This puts more load on the outer axle bearing. It also creates a weak point.
I say this because of my experience with people doing it. I have never seen a sprinter axle but, I'm sure its the same. The people who sell you the kit don't care as long as you buy their product.
I have a sprinter in my group and it looks like its lifted a little. Not sure who's kit he used. You should research the lift's and see how far you can lift before you strain the front axle and cant align it. Most vehicles will go 1-2 inch before any issues.