Unless newer astros have multiple leaf springs- you just have one composite leaf on each side in the rear.
Helper springs can be had for like $60 bucks, Helper bags for something like $300.
http://www.suspensionconnection.com/cgi-bin/suscon/results_by_vehicle.html
You can check out my build (link in the sig)- but I have a wall at the C-Pillar. I have the STOCK suspension. No extra anything. It has dropped it like 2.25" in the rear, front didn't move. I would like to get the helper bags, but $$ might mean helper springs. ...As my van isn't daily anyway... BUT- yeah, beefier brakes aren't a bad Idea at all. LOL!! I noticed a HUUGE braking problem after the wall went in. Yeah, don't let anything jump out in front of you...