1. Rust! its a unibody with very few patch panels available. Look at the roof rails, the floors (especially in front where they installed a rubber floor mat), and along the bottom of the body.
2. Body damage. See No.1 unless you are a body wiz.
3. Wiring. 40+ year old wiring and a cheap car = hack fixes. No replacement harnesses available.
4. Complete and all parts there. Mostly dirt simple mechanicals, but a few items are hard to find. Steering boxes, one tie rod end, Ball and trunion driveshaft, gas tank. It's often easier to find a parts van than to track down all the little missing stuff.
Head over to vintage-vans for more info, there is also a Yahoo A-100 group.
They are almost all manual steering, all manual drum brake, very rare A/C. All are either /6 (170 or 225) or small block (273 or 318), either 3 speed on the tree or 3 speed auto (all 727) on the dash.
Minor differences between 64-66 and 67-70, mainly safety.
Figure out what drivetrain you want and what body style.
Here's a decent allpar article and a wiki article covering the basics.
http://www.allpar.com/trucks/vans.htmlhttp://en.wikipedia.org/wiki/Dodge_A100
