To begin with, if you are looking for a rifle that will accept a scope, then you need to find one that has rail segments on the top rail making it more useful for mounting optics. Some airsoft rifles allow the addition of rail segments that you can purchase individually, but having these rail segments already installed on the gun will make things easier for you.
Are you looking for a spring rifle or an AEG rifle? Spring rifles normally are less expensive but shoot the ammunition at a slower speed. These are great for the beginner or for the younger enthusiasts. AEG( automatic electric gun) rifles are more expensive generally, but the velocity of the ammo is greatly increased. The gun is powered by a rechargeable battery.
Full metal airsoft rifles will be a lot more reliable than their plastic counterparts, and also give you the feeling that you are firing the real 303 British ammo thing. Plastic rifles are still a great choice and offer the same velocities of a full metal gun, but the chances of it breaking if it is dropped are more likely.
You will want to make sure that the magazine that holds the ammo is large enough to hold a huge surplus of ammunition. You will not want to run empty and trying to reload as your enemies are approaching. Also, it is best if you choose a gun that will fire the ammo at a velocity of at least 325 feet per second as this will greatly increase the ability to reach your intended target.
If you are choosing an airsoft rifle that has a gearbox, then make sure that the gears and the bushings are metal. This will insure that your gun will perform at a high level for years to come.