The clear G&G stuff is decent entry level from what I've heard, but the great thing about black rifles are that they retain their value...
If you spend 500 or so on a used black rifle you'll really save a bit of money in the end... G&G's normally go for what, 380 or so? Add another 250 to 350 on for a decent metal body (don't bother with Dboys... those bodies are awful) and you're over what it would cost you for a good name brand used gun.
Black rifles also retain their value alot better than clear guns... you may play the game only to find you don't like it... If you're gonna go clear I'd suggest a cheap Kraken from Cabelas (I think they're only like 115 bucks or something right now)
But if you're set on an M4 and don't mind the clear lower, go right ahead and buy one... But when in the future you want to upgrade the body, just sell off your clear G&G and buy an all black gun (It's cheaper that way)