Glenn, excellent question and one that all people wanting to get into big track events struggles with, I know I did.
First I want to say check out Rennlist hisotyr on this topic. Lot's of good threads about this; however, here is a recap of some of your options as I can recall them:
1. Drive on street tires to event and get someone else to bring your wheels/tires to the evet for you.
Pros: You don't have to buy or rent anything to transport
Pros: You can use any track tire.
Cons: You will likely have to bribe, beg, or something worse (can't mention it here!) to get people to continually transport your tires.
Cons: If your car breaks down tow backs are tough.
2. You use a more suitable street/tack tire on than Kumhos lke the Michelin Pilot Sport cups or the Dunlop Super Sport race. These will handle the street better than Kumho over long distances.
Pros: You only need on set of wheels/tires
Cons: These tires in 18" are around $1200 - $1300.
Cons: If you have a tire or car failure you are out of the event or need a tow back.
3. You can put a hitch on your car to carry wheels.
Pros: you don't have to beg for tansport
Pros: you can use any track tire.
Pros: easier to carry tools as well.
Cons: You need to put a hitch on the 993.
Cons: If your car breaks down you new to tow the car and the trailer.
4. You can rent a trailer.
Pros: you do not need to store it when not in use.
Pros: It is relatively inexpensive to rent.
Cons: you need a tow vehicle and assorted tie downs (don't use the chains that come with these trailers)
Cons: Sometimes it is difficult to rent them if you do not reserve ahead of time (El Camino rentals has a nice tilting bed model)
Cons: if you do not rent from the same place doing the hitch and loading will be a learned experience each time.
5. Buy a trailer. You also need to decide if it is an open or enclosed trailer.
Pros: you always have a trailer and have the set up and loading down.
Pros: you can go to events on short notice....
Cons: You need to buy it....money
Cons: You need to store it.....could be more money
Cons: you need to maintain it....some money
6. You could do an arrive and drive set up like Tim C!
7. Your could abandon the idea and just auto-x
I am sure this will spur on other comments and questions so bring them on!