A wastegate does nothing for Lag. It is there to bypass exhaust to limit excess boost on the top end. The reason the van turbo does not have one is due to the large exhaust housing. It will limit total boost by nature. Now, the larger housing will in theory make it lag more than your stock one. How much, I do not know.
It will bolt directly on, but you will need an exhaust adapter. I believe Bean's Diesel has them, Diesel Innovations used to, may still.
From what I've heard these are great for towing as they will help lower EGT's and help with surge. Some add the Wicked Wheel and/or the ATS ported compressor housing to help with surge also. Those two mods will also increase the whistle if you like that.