New headlights are available, avoid the autopal ones. Matt probably has some good s/h ones. I think VolksBaru might have some.
The seat belt issue has complications. It's actually illegal for wreckers to sell second hand seatbelts in Victoria. You could buy a brand new universal seatbelt but it MUST be retractable to pass the roadworthy, the trouble with this is that depending on the year of your Golf you may not have the mount on the tunnel to bolt the stalk to. The mount can be easily fabricated if need be, but must meet certain criteria. I can assist here if you chose this option.
I think that the rear retractors from a SEAT will mount in the front of a Golf with the front SEAT buckles off the SEAT seats, but I haven't tried it yet.
(She sells SEAT seats by the SEAT shore?):)
I'd suggest borrowing a belt for the roadworthy but you really need to do it properly - they might just save you life one day.
For the boot seal wander down to Clarke rubber and have a look at their pinch weld sections, I'm sure you'll find something that will do the job. Otherwise you'll have to order one. Remember to measure how much you'll need before you go and get some black silastic and a round rubber offcut to make the join.
Pete