That question of what is a good gun gets asked quite often on this website and for the most part everyone has their own opinion on whats good and what is a good price.
Its kinda like walking into a grocery store and asking the clerk "whats good?"
You need to decide what you want first. You've stated that your interested in assault rifle of some form. Lets examine that in a little more depth and add some more information. Is there a specific make or model of a firearm you would like to have? Do you have an interest in the requirement of rails so that you can add accessories?
Decide what make/model or airsoft gun you would like first.
I'll provide a few examples for you to get your brain juices flowing:
Armalites: M4, M16, C7
Automat kalashnikov or AK: AK47, AK74, AK 101
G36 series: G36c, G36k
SCAR: Scar L, Scar H
When you have selected or come up with something maybe then we can proceed with assisting you with some suggestions.
"Its only a little bit on fire"