Good reply 97 black lude
Ok here goes.
1) the 7.5" is referring to the width of the rim in inches.
2) Offset is the measurement from the lip of the rim to the part that mounts it to the car. Kinda hard to explain without pictures. Just get an offset between 44-50 and you'll be fine.
3) Getting an aftermarket header will allow the exhaust gases to flow more efficently. It will give you a increase in HP's, but not enough for the price IMO. Get an Intake first. AEM Cold Air Intakes are very nice.
4) A cat-back exhaust system replaces all the piping from your cat and back. Just a muffler will only replace the muffler.
5) Yes, you are a super noob. It's ok though everyone of us was a noob at one point. However some learned sooner than others.
Welcome to the board

hope you stick around