Is the car fixed (i.e. no major body damage)? $500 is a pretty good deal.
1. A tranny swap depends, like said, but you're looking at $2000-3000 inc. labor.
2. Anywhere from $150-$600 for exhausts. Custom exhausts can run just $150-$200. For the intake, buy an eBay CAI and get an AEM DryFlow filter... Trust me, same performance for a fraction of the price. If you don't believe me, check out
www.newcelica.org.
3. 5x100 is the bolt pattern. The Celica likes +35 to +42 offsets. Don't get any others.
If you need more help, head out to NewCelica.