1999 VW Caddy Questions

[Waddellg]

Hi,
I've inherited a rather worn 1999 Caddy diesel from my father in law. My son wants to fix it in memory of his grandfather.
I have a few questions

1. The fuel flap was broken off and there is a non-locking cap - I've ordered a replacement locking cap off of Ebay but can't find any Caddy flaps - can a Polo one fit?
2. The front door locks have been tampered with the drivers side does not lock either with the key or internally with the button. Passenger side locks but only internally with button external handle is loose. Can I use Polo parts to fix?
3. Main back door lock does not work either, but top latch does not catch. It appears that someone stuck something between the frame and door to break in will I have to replace the door? or can it be fixed?
4. The seats are all worn again as Polo's are more common, can I swap in Polo ones?
5. Finally I know it is a diesel and definately not a TDi but is it a SDi or an earlier engine? How can I tell? No badges and haven't found registration docs yet. It is slower than my 1.4 TDi Seat.

[Waddellg]

OK,
I fixed the rear door issue by "remodeling" the catch on the frame top with a hammer now the top latch closes but door lock is seized...
Going to have to find a Caddy or Seat Inca in the scrapyard and get the locks off of it.