I would not recommend buying a dboys, jg, or lancertactical. Go to Airsoftgi.com and type "Custom" into the search box and hit enter. It should give you a list of customized full metal airsoft guns with varying prices. Make sure that you buy a Full metal AEG or GBB rifle.
You will find many decent airsoft pistols for ~$50 but nothing more. Don't buy an AEG under $140 or so.
You can buy an Electric Airsoft Gun (AEG) online or at a sporting goods store.
A good electric airsoft would be a SRC Gen. 2 or 3 XM8..
Not bad
The g&g gr16 is a good AEG for its price. It is reliable, and an all around balanced gun.
You will lose velocity
I would get the JG MP5 SD6 Airsoft Rifle AEG Gun 067. It has excellent performance and is great for a beginner.
I look at a few things, but mostly I check out reviews of the gun. There's many good airsoft gun review sites. Then, I look at who makes the gun. Some companies are known for making good guns, and others for making crap guns. I would try to find a good review of the gun that shows what's in the gearbox. I wouldn't buy an AEG unless it is over $100. Don't buy Crosman or Boyi.
The 'M4 3181 AEG FPS-300 Electric Airsoft Rifle, M203 Spring Grenade Launcher' is a HOT airsoft rifle. Not only does it offer a high power electric machine gun that shoots full or semi auto, it also has a grenade launcher. It is priced around $90, not bad for a rifle and grenade launcher.
Automatic Electric Gun - i.e. airsoft toys guns.
my Willy wonker aka mm nan
If you just bought the gun there is really no need to buy new internal parts.