I wouldn't spend any $ on an axle or gear swap until you see how it does. Unless you're looking to max out the towing capacity of the van, it will do fine with lighter trailers and your intended uses (shorter trips you said). Yes deep gears will help but that's a lot of budget for the return imo unless you're logging a lot of miles or flogging it with a big trailer.
Springs shouldn't be an issue in a 3500. Doubt you need bags. Hitch, brake controller and an aux trans cooler if you're inclined and let er rip.
