I think why some may recommend staying away from M16 family of guns is because of the V2 gearbox. Of all the gearbox designs, it's the weakest neat the cylinder head, where the shell absorbs most of the impact from the piston. While it's true that they usually won't last as long as other gearbox models, a good gun with a reinforced gearbox shouldn't give you any trouble for a long time as long as you don't equip it with an über-spring and maintain reasonable velocities.
Personally, I think if you're used to real steel Armalites, then they're also excellent guns. As already mentioned, they're easy to work on, they have tons of accessories readily available (from cheap to ridiculously expensive - whatever suits your budget), and mags are readily available.
I will also echo what Kuro_Neko said and recommend a Classic Army M15A4 of some flavour or another. That way, you'll be starting off with an all metal gun with a high quality metal body, a reinforced gearbox, and reinforced gears. The external plastic parts are some oif the best you can get in a stock airsoft gun, being fibre-reinforced nylon instead of the weaker ABS plastic used by most other manufacturers. Stock they shoot at pretty modest velocities, but that's not a bad thing when you're starting out. And adding a new spring to up that velocity is pretty simple.
Also, something that wasn't mentioned - get yourself a good battery and charger. Get yourself a good quality high capacity. www.cheapbatterypacks.com
is a good place to shop for quality batteries at a reasonable price. If your gun comes with one of those wall wart chargers, promptly toss that shit in the garbage, as it will kill your battery.
Get yourself a good smart charger. It will properly charge your battery and keep it in top condition. If you get a Ni-Cad battery, either get a discharger or make sure your charger has a discharging function. Ni-MH batteries don't need to be discharged before charging, so if you go that route, you don't need a discharger.
And as already mentioned, make arrangements to meet with an age verifier. Once you have your AV status, you can then have access to the classified and retailer sections of ASC, where you'll find ample new and used (sometimes well upgraded guns) that will easily fit within your budget.