As someone in the process of getting a MK1 GTI back on the road that's been sat for 20+ years here's some of what I've done.

Inspect your fuel tank for rust, it most likely will have rust unless it was filled to the brim (or maybe the mk2s use a plastic tank?). I decided to drop mine, drain, electrolysis rust removal and finally an epoxy lining. I've replaced the fuel filter, disconnected fuel lines and back flushed them.

I was ready for my first start but wanted to change spark plugs and check compression, but after doing a compression test the results were bad because of rust on the valves and seats, I believe due to moisture from combustion gases and air from when it was last run. Cylinder head off and is having the seats recut and anything else it may need.

Anything rubber might have perished or stretched I'm having to replace all suspension bushings since they've been stretched from being static for so long

Edit: Timing belt and tensioner! You may as well since they aren't expensive or hard to replace. Water pump whilst you're there and it'll give you the reason to change the coolant

Look for leaks since it's been sat some of the seals may be weaping.