All your questions make sense but could set fire to the forum at some point. Wanna bet with me?
You can start with the 1100 if both versions are perfectly identical, in and out. This is a new kind of bent car, the displacement being smaller! The 1300 is rather easy to modify, all the good parts/tricks and suppliers are known. If you go for a category where the manifold is free, go for it. If not, it must look like the picture on the form, including ID if specified.
To start you've better study appendix K drawings about ROP's, nobody will accept anything different anywhere on earth. Or you can go for a recognized supplier, may be Custom Cages or Safety Devices. In this case, take care because some have a slight tendency to weld the cage to you car forgetting that you have to put the dashboard back afterwards! Or the headlining… Or the seats…
Even if you dont apply at the moment (why would you?) if you're looking for entries in any FIA event, the HTP is mandatory. Always. Specially when this instance is the organiser. Choosing the right route will save money and time.