First of all, let me confirm... Darman's neck seal is great!
When you strap your armor together, you can make snap plates either out of spare ABS or nylon webbing. I went the webbing route, and haven't had any problems yet. It's cheap, so pick up a few yards of 1" webbing. It's also good for making a garter system to hold up your thighs. As for snaps, the popular brands are either Dritz or Tandy. I used both on mine, and found that the Tandy tended to hold a little more secure, but they were harder to install. Just when you think you have enough snaps.... you'll need about 50 more. In other words, get A LOT of them. You also need a few yards of black elastic for the strapping, and a little bit of white elastic for the chest-to-back straps.
Also, you'll need rivets for your thigh ammo pack and belt, as well as a bunch of stuff to finish the bucket: either stickers or paints for the tears/traps/tube stripes, lenses, either foam or a hard hat liner for the inside, accurate mic tips (the ones included in the kit aren't quite right). You could also look into the electronics, like a voice amp and fan kit.
Plus, all of the tools mentioned elsewhere!