what kind of motors/trans can I fit in a gt1

OK...I will be the party pooper...again!

Don't invest much of your money into this car. You will likely be unable to finish it and will be forced to sell it for much less than you have in it.

That being said.....these kits are hard enough (well not really...) to build in the stock configuration, especially for someone who has no strong mechanical skills. Remember...you are building a CAR. Not a toy. Not something to bang around in the woods with, but rather a vehicle that will carry your lie in it at deadly speeds.

So...hit your local classified and Craigslist and try to find a stock VW motor and transmission to use. You will need to know if you have a IRS or swing axle pan. Yes..this is confusing but do some searches (The Samba, Google, etc) to understand the differences. Everything else will be WAY beyond your abilities. Try to get a running one for as little money as possible. If you find something post it here and people will give you feedback.

Now...onto your questions:
Tail Lights: You can use whatever you want. If you want stock then go to your local NAPA and get the lamps, the pigtails, and the rubber mounting gaskets.
AC: To expensive and too complex for you.
Seat Covers: You will need to have them custom made unless you can find some on the classifieds here or maybe EBAY. Like custom and likely $300-$500 (its expensive building a car)
Radio: The absolute least of your worries. Head to Walmart and find something you like.

Here some other hints as well:
1. Check your wiring. I have rarely seen an abused and abandoned kit that had the wiring done right. You will likely need to gut it and re-wire. That will be about $400 if you can do it all yourself.
2. Brakes: They will likely need to be rebuilt and new lines run. Good news though...they are cheap at about $200 for all 4 wheels plus maybe another $50 for hard lines.
3. A runnable motor and trans will likely be in the $500-$600 if you are lucky.
4. If it has been sitting for a while you will likely need to have your gas tank cleaned (if it is not rotted) and the lines either cleaned out or replaced. Probably another $100 if the tank is solid (and they usually are).

If you have anything else missing let me know and I will be happy to give you an estimate for the cost.

I have been fliping scooters and such for a couple of years now and that is what I know so any info on this is helpful.