I have a 3" lift under my GU and its a great height. Enough room to run 35s without too many issues. Doesnt stand out. Enough lift to get the drop boxes in as a 2" lift your stuck with bushes only. You dont need to change rear trailing arms at all on a 3". I would change out the lowers for HD standard length though which i have done. You can get away with standard panards to if you want.
4" then absolutely do need adjustable panards front and rear and Upper trailing arm adjustment added to the list.

