On a stock car the 2.5" collector of the JDM ITR header isn't going to produce much of a benefit. If I was in your position I would try and swing a DC Sports 4-1 stainless header. They run about $365 but maybe you can chip in some money to your parents to help with the cause. Otherwise the ceramic-coated mild steel ones run about $300 and are the same design with the not as high quality piping. I would pop for the stainless though.

For a cat you can get a Carsound high flow cat with all of the flanges and oxygen sensor plugs welded on from ipsracing.com for $185. Make sure you order the one for the 2.25" collector.

If you can only afford one thing for now you might as well get the header and then put the cat on your to-do list.