Hi Naja002, and welcome to SimViation

Acceleration is en expansion pack as you said. It's called Acceleration (the original name was Adrenalyne, changed before shipping to the stores) because it contains a very nice Hornet, a Reno-racing style Mustang, and a nice Reno-racing system with several tracks to race.
You do not NEED it, but as a newbie OR experimeted flyer, you might want it in order to enjoy:
- carrier operations with the Hornet (or any aircraft that is configured correctly)
- air races
- new missions
- 3 nice aircrafts: Hornet, Mustang (racing modified version, not the original WWII type) and Merlin helicopter
- and some nice other features
You will however have enough to discover by simply restraining yourself to the original FSX with its two official free patches (SP1 and SP2, both also included in Acceleration).
I'll leave the rest of the answers to the experts.