So what are you? Track or Road? You seem to have put a foot in each and made a confusing spec.
First, I would never have a cage for road. You must always wear a helmet if you have a cage. The consequences of clouting your head are major. Do you really want to wear a helmet all the time on the road?
Second, three point belts and a cage? Thats just going to make the impact off the first point worse.
Third, does the windscreen fit that track day cage? I am just looking at my UK 420R photos and I think it will conflict with the mounting points of the windscreen. You should probably confirm you are ok with your Caterham dealer. Sidescreens will work with that cage if the windscreen can.
Fourth, I do not see it but add an LSD.
A couple of other things:
1) what suspension set up are you spec'ing? The standard R version?
2) The ITB upgrade to the standard 420R plenum has caught people out with the ECU upgrade. It always seems to need to go back to the factory to get reflashed - check with the dealer to make sure it is done prior to arriving in your hot little hands. Its an essential upgrade once sorted - sensational induction noises.