Deffo putting seats & harnesses in but not sure about a cage yet?
To be done right, the harnesses need to be secured at shoulder height. You can only do this properly with a cage with harness bars in it.
As such, it's either do all, or do none.
Stripped the bars from the doors last night but left the window motors in just now for ventilation, will probably remove them at some point and fit foamalux or perspex to cover the holes.
The foamalux is to replace the doorcards.
You need polycarbonate/Lexan to replace the windows. Perspex shatters into shards and is nasty.
Its got a sun roof so thought about ditching that and filling with perspex too, how hard would it to do the same to the rear side and back window's as there bonded in?
Remove sunroof and mechanism, replace with alloy sheet, bonded with Sikaflex or Tigerseal and rivetted in.
Rear windows can be removed and then you can use Sikaflex or Tigerseal to bond in polycarb items to replace. Same should be possible for the tailgate.
Talk to these guys about securing the rear windows and tailgate:
http://www.acwmotorsportplastics.co.uk/ They were very helpful when I got mine.