If you decide on the M4 route, here's some more info. Since I'm a big Armalite fan I know more about them.
If you're willing to spend up to $300 you could get a JG. Yeah JG's are China clones but they're getting quite respectable nowadays. JG's M4 runs about $290. You could get one of the G&G cansoft (clear receiver) M4's for about $340. G&G is the cheapest of the midgrade guns. $400-$450 would get you an ICS which is a higher end midgrade gun. $450-$550 would get you a G&P, there's some debate on whether this is the highest of the midgrade guns or the lowest of the higrade guns. Either way they make nice guns.
If you can afford it highgrade guns such as Tokyo Marui or Classic Army are the way to go. They're going to run you $550-$700. Though if the M4/M16 style is what you want then I'd recommend staying away from stock TM's, the barrel wobble and creaking are pretty bad on the stock TM M4/M16's.
I did extensive research and settled on the Tactical Carbine version of the Classic Army M15A4 as my first gun. If an Armalite is what you're looking for then I think it's the best of all possible versions. You have the shortened (but still decent length) M4 carbine barrel, which makes you less likely to smack your barrel off something when turning quickly, combined with the full stock which houses a large battery. Small batteries don't have near the run time of a large battery, not to mention that the reinforced slip ring (the ring that keeps the fore-grip on) is really stong. While this is mostly a good thing and a big step up over CA's previous M15 versions, it makes changing small batteries in the field very difficult if not impossible.
CA's M15A4 series in general is a big step up over their previous versions and as far as stock armalites go they're probably the best. All metal where the real steel version is, the plastic parts where the real steal version is plastic are made of high quality enhanced nylon fiber rather then abs for great texture and feel. Classic Army has a deal with Armalite so you get all the proper markings. Plus that deal also goes to show the quality of the manufacture, Armalite wouldn't put their name on a shoddy product.
Internally the gearbox is reinforced metal and all the gears are metal as well. It fires hotter stock then Tokyo Marui's as well, firing at 300-320 fps where your average TM only fires 280-300 fps.
All in all this is a great starter gun if you can afford it. Because it's all metal you don't have to worry nearly as much about breaking it and you won't get any of the wobble or creaking that is infamous in stock TM M4/M16's. Unfortunately affording it is the biggest problem, they're not cheap. For reference I got my CA M15A4 from the classifieds, upgraded with a bunch of Systema internals to fire 390fps, for $600 shipped which was a great price.
One of the perks of going with an M16/M4 variant is that mags are dead easy to come by and dirt cheap. You can get Star brand mags for like $5 a mag or less, plastic yeah but sturdy none the less (Star is also the only brand that makes realcaps too). Metal mags will run you a fair bit more, $15 to $30 a mag. There's no real reason to go with metal over plastic for mags except looks/realism. Though of course if you're going to go hicap then you might as well get metal ones. As to Brands, TM and CA make good mags. I've heard pretty good stuff about MAG brand mags as well (though recently several of my friends' MAG mp5 mags have been spliting down the seams, don't know if this also affects m4 mags). King Arms not so much, especially for M4/M16 mags. Star is always a good affordable fallback too.
Now for alittle Gun pron:
When I got it:
edit: I agree with Saint that m4 pouches are much easier to get, mp5 pouches are a pain in the ass to find. That said, I have a MP5k for CQB and I've found that a pair of mp5 mags mag-clamped together fit great in most m4 pouches.