My thoughts.... First, not every knife/design is a good candidate for kydex. Kydex holds through pressure points. Ideally that point should be the plunge cuts for the least wear and the longest lasting sheaths. Knives with large guards, and those with significantly dropped blades in relationship to the handle are some that simply have kydex sheaths.
Next, .093" thickness is the most common, generally the least expensive, and makes about the best compromise between moldability and long term sheath durability. Whatever you do, DO NOT buy your kydex by the foot....from knife supply outfits! If you do, it will cost you approx 4X what it will cost per sq foot if you buy a full 4x8' sheet from somewhere such as Port Plastics (they have multiple locations around the country).
For .093 kydex, you want the 8-9 eyelets Brad posted the link to. Just in case folks don't know.... the same folks own/run KnifeKits.com, so check prices on both....they are usually the same, but often you can catch a sale on one, that is not on the other.
Post screw length depends on the layers of kydex your going through. Typically I buy/use Tek-Loks for belt carry..... that come with the hardware. Otherwise I use Eyelets to "assemble" kydex.