I would recommend you try and get age verified if you can. I skimmed the classifieds and noticed there are quite a few M4s that meet your requirements (full metal, etc) and upgraded price range.
Many of these have extras that you'll probably be wanting. Mags, batteries, foregrips, supressors, sights as well as internal upgrades for higher fps/accuracy/precision.
M4/M16 models are the most popular out there. Its definitely worth a shot. You'll be surprised with the amount of choices you find.
H&K G36c (KWA) / Glock 18c (TM) / Kimber LAPD SWAT Custom (TM) / Mossberg M500 (ACM)