possible answers
1) Yea, well, they are great cars- where did you get them?
2) The DP-08 says they were mfg'd by Elan, not Van Diemen, and were early cars so probably around 2012 or 2013? I have 006.
3) Since they were not SCCA Club cars, there are no log books. You will need to one at a SCCA race if you enter one.
4) That handle is the neutral/reverse lockout. You will need to pull it to go into neutral from first gear and then again to go into reverse.
5) Gas cap- call Primus Parts
6, 7 and beyond- rip the entire data system out of the car and replace it with either AIM or Motec. It's old, outdated and there is no modern software for it. (If you really like it, I have two more complete systems for you sitting in a box). My suggestion is to call Veracity Data and order through them- Dave is a wizard in installation in these cars.
What you haven't asked yet: what map and restrictor is in the cars? In a perfect world, call Zentec Wiring (California), order new chassis harnesses and replace the Pectel ECU with a Performance Electronics PE3, or at least replace the Pectel and get a jumper to the old wiring system. The Pectel is also basically outdated and cannot be repaired when it breaks, but you can probably sell it fairly quickly.