To tow the van you only NEED the suspension upgrade and a weight distribution hitch (check that your tow hitch supports this first - it has to have a LOT of bolts in it). You don't NEED anything else in that list. None of it.
There are a few hills that give our rig some grief, and the trick is to just let it down a gear or two until she's no longer struggling. Allow the gearbox to amplify the engine's torque and you'll make it without any worries. You might get there 90 seconds later, but I'd rather spend 90 seconds more behind the wheel than 2 days waiting for a part.
Airbags schmairbags. You do NOT need them. You CAN use them, you CAN do it safely, but dual-rate springs will win over airbags every single time - because they won't inhibit movement, and they place the load of the vehicle precisely where the chassis was designed to take it. Don't let a salesman tell you otherwise, because he won't help you while you're on the CSR with a bent chassis.
You do not NEED a snorkel. It's great to have one, but you don't have to bust a nut getting your hands on one. If you DO go for one, consider the cost difference between a cheap one made from inferior plastic that disintegrates in the sun and the slightly dearer ones that you'd normally see (Airtec or Safari - Nissan use Airtec, TJM sells Airtec). I think they're a good idea - they allow cleaner, cooler air into the engine than you'd otherwise get - but the car will drive just fine without it.
The only thing you actually SHOULD change is the rear suspension. Don't bother with helper springs (eg Ironman Load Plus). They're good as a temporary fix, but long term they can't handle the weight.
Oh, there is one other investment you should seriously consider. Camps 7. Don't look here, don't look at Nissan. Look in Rays Outdoors/BCF or other camping stores. It's a gold mine of places to park the van, both free and otherwise.