Re: AR-15 question
As far as brand and model of quad rails you have a lot to choose from. I personally like Yankee Hill, but their are a lot out there. So do your research.
A non free-floating rail is easy to install. You can do it yourself. Just push down your delta ring and pull the old one out. Then just fit in the top and bottom of your new quad rail. Tighten the screws and your good.
A free float rail you might not be able to do yourself ( I say may because I don't know if your a machinest or handy, or maybe have acess to the tools needed). You may want to consult a gunsmith on a free float. The free float will give you slightly more accuracy, but unless you plan on shooting comp I would worrie about it. Regaurdless, putting on a free float involved removing your A2 gas block (your front sight) which I wouldn't dabble in unless you know what your doing.