There are a lot of parts available for your GTI from Purring Pumas, www.purringpumas.com. Original seats will be almost impossible, but take heart, there are a lot of inexpensive seat options available, such as SCAT seats, www.scatenterprises.com. Just remember that you want a seat with a thin bottom cushion. It's much easer to raise a seat than it is to lower one.
Purring Pumas has all of the rubber seals available, and Allen and Alfredo are great to work with.
As far as I know, there are no spoilers available for Pumas in the US. Original registrations for the cars are a bit iffy...the year is often the year the car is first registered. The GTI and GTC have a vestigial front air dam molded into the body, which would require a great deal of work to bolt up a larger air dam to. However, since the car is fiberglass, it is not that hard to modify permanently.
For the record, I extended the air dam on mine this summer with duct tape and plastic to play with, and did not notice much difference at 80 MPH. My car is a GTC, however, and not particularly clean aerodynamically. A GTI should be much better.
Do you know if your car has been lowered?
Another treasure trove of information is Brandon's www.pumagte.com, which may be the best website out there for photos, articles, and information on the Puma.
Congrats on your new purchase !!! One tip I can give is that if you do order parts from purring pumas, have them ship it by USPS not UPS as their duty/custom charges will make you puke fast enough. I have a 1981 GTC and I live in MTL, Quebec and trust me I've learnt the hard way.... Don't worry about dealing with purring pumas, they are true professionals and very nice guys too.... Come see us at www.pumagte.com, Brandons' website is very informative and well made (you owe me ten bucks Brandon...lol).