Not familiar with them myself, but I am looking at a simmions sale catalog for this month and they have a M&P15 811003 Optic Ready 5.56 NA70 for sale for $850.
What Mr C said:biggrin:Get one marked 5.56. The 5.56 is loaded to a higher pressure than the 223 Remington. That way you'll be able to shoot either ammo through it. The chambers for 5.56 is slightly different from the 223 chamber. The 5.56 is cut with a longer leade. Here's some good reading on this subject: http://www.winchester.com/lawenforcement/news/newsview.aspx?storyid=11
Also, make sure the barrel is marked 5.56, it doesn't matter what 's on the lower receiver. The barrel will usually be marked on the top of the barrel, in front of the front sight post. For instance, a Colt barrel will be marked something like this: C MP 556 1/7. Here's some info on reading and understanding barrel markings: http://www.biggerhammer.net/ar15/barrelsbolts/.
As far brands, as long as it meets spec, get the one that your wallet can afford. Colt will by far be the most expensive, but lots of people think (rightly so) that Colt has better QC than the rest of the brands. Right now, Bushy is probably the most popular. Others to consider are Rock River and DPMS.