I personally have a KWA M4 and I absolutely love it. It's on the Higher end of the non-Systema M4's both in price and Quality. I think I paid around $550ish for it new. Battery wise, I recommend LiPo but with some guns you'll have to upgrade some of the gearbox parts to be able to handle the higher outputs of the battery. That's a big reason I got the KWA was it's beefy enough to handle LiPo right out of the box. That being said I've heard good things about the G&P's and the King Arms. There is also the less expensive and common route that people take and that is the JG brand. I've heard good and bad reviews of them but mostly good and around the $250-350ish mark you can't really go wrong, especially as a starter gun.