You'll likely need an armorers tool to remove and replace a barrel nut, but this depends on the kind of quad rail you get. Some of the 2 piece designs are made to replace the old school plastic hand guards.
I have seen about 3 basic types.
1.) Round tube with attached rails.
2.) Rails milled into the tube.
3.) 2 piece clam shell tube that fits together over the barrel/gas tube.
The kind you get is up to you, but you can find quite a few for less than $150. The next time I get one, it will have the built in flush cups into the rail, it will just save me from needing to buy another part to mount a sling.
The new half rail for ends that RRA has are nice, but I would rather have full length rails that I can put rail guards on. Putting your hand directly on the aluminum tube get's a little cold for my liking.
As for the length, most of the rifle length quad rails are going to be 12.5" but you can get longer ones that will extend over your gas block. (you may need a low pro gas block to utilize them though)
Places like this specialize in lots of stuff from many makers.
Midwest Industries
Bravo Company
M & A parts
Lots of reputable AR builders have their own as well.
I would hate to guess how many different kinds of railed hand guards you can get. Thousands.
