The one you can afford
Nerf gun
The best airsoft gun you could get for $150 would probably be an Aftermath gun, but you could get a JG M4 or M16 for under $150
If you are just starting airsoft i would highly recommend an aeg, (airsoft electric gun) but if you are pro and experienced, a trained camouflaged sniper is the best, an invisible predator. However it does depend on the user of the gun. If you are asking performance wise then g&g makes the best airsoft guns on the market. (currently in the US)
dont get that gun walmart has the best beginer guns
I say twin ozez are the best cheap airsoft guns
www.shortyusa.com
Definitely Nerf.
it depends on what cheap is to you
The best way - go to near airsoft workshop, give them money and tell what do you want. If you decide to modify airsoft gun by yourself - be warned, this may ruin your gun and even be dangerous to your health.
"Systema" is by far the best airsoft gun brand out there.
An Airsoft gun
A pump airsoft gun is a gun that has to be pumped every time before shooting. Ex. Airsoft shotgun