Good luck with the car:~
I have the '88 GL Estate with similar mileage: never found anything on the car that is beyond the average DIY fix.
Seats will come up well, just take some time on them- all materials are reasonable priced
https://www.woolies-trim.co.uk/ https://www.carcarpets.co.uk/
https://www.segalmotortrimmings.net/
The above are just a few descent suppliers but online ebay is also useful.
FIO I believe the foam is remanufactured in the USA (quite a complicated shape) but no need to go that far for a competent repair.
The tailgate wiring is also quite straight forward: you can do a side at a time by supporting the boot door and removing the appropriate hinge- you will find a connector on the left hand side (looking at the boot) just under the headlining, splicing in new cable as required - shrink down sleeving is also handy for the flexing through the hinges.
Regards Bob.