Hi So1o,
Welcome to the site, looks like a quality car you have there

Arches: where they will have rolled the metal it leaves nothing for the rivets to fit into that would have held the plastic arch on. You need to bond them to the body instead. The bodyshop will know what to use.
Handles: the 'chinese' ones are a very new thing, most likely modified mk2 rear door handles. mk3 handles are not an easy swap. If it's going in the bodyshop how about early or late Audi handles? It's a common swap that does need a lot of cutting and welding but it looks great. Or just stick to rear mk2 ones for ease.
trim: I would probably keep the door trim. The arches have a section that is connected with the bit on the front wing. That would need cutting and smoothing to keep tidy.
Other ideas: transporter wing vent cut into the wing - pretty common but looks good. TT fuel cap cut into place - big mods but would look trick
jv
