Hi Paul,
Looking at your set up in the pics, it looks like the previous owner was keen on the small bumber look, your best bet would be to aquire a full set of early door run strips, they are slighty wider/thicker then the post 90 items, if you can get a set of the early arches it will finish it all off bices as they meet nicely from the front wing and also the rear quarter, if your looking to convert back to the post 90 big bumper look then again you need the bumpers/side door mouldings/sill covers/arch trims, but like you say the side door mouldings are very hard to source now.
Hope this helps..
PS. the early side door mouldings did not have a recess for the red gti badge or a badge.
Tesh