What tires to buy?

[VRONA@ix.netcom.com (Steven Verona)]

I recently purchased an 86 4k Quattro that I use as a daily driver and 
for Solo II racing.  The guy that had the car before me was really cheap 
and put on some no-name 175 size tires.  Keeping in mind that the car is 
used everyday and I can't afford a second set of tires and wheels:

1.  What model tire should I buy to maximize racing performance?
2.  In stock class, what is the maximum size tire I can legally run?
3.  Should I use the widest tire or is there an advantage to narrower 
ones?
4.  What other changes should I consider to make the car more 
competitve?

Thanks for your help.

Steven Verona
86 Audi 4000 Quattro
Race record: won 2 out of 2