The barrel is the 16in and I was thinking of the forend being a few inches off of the end of the barrel. Is there a gas block I could get that could be covered by the forned?
Absolutely. Get a low profile gas block and it should fit under the forend. Double check dimensions before ordering to be sure.
Do I really need the front sight?
Need it? No...it depends on what sighting system you're going to use. Red dot? ACOG? Irons? If you're going the red dot route, you can do without front and rear sights. Most guys like them in case the primary sighting system goes down (batteries, malfunction, etc). It also depends on what you're building the gun for...a real fighting gun? You need them. A plinker or a budget build, you can do without. Some people keep the standard front sight and only put a flip up rear sight on.
Co-witnessing the iron sights through the red dot is a popular option.
I was wondering if I need something on the front of the forend to hold it on.
Hold what on? The front sight? Most flip up sights attach directly to a section of picatinny rail. It depends on what type of handguard you purchase. If it's a railed handguard (Larue, DD, etc), you just attach the front sight to the rail. Those types of handguards are called free float because they're only attached at the upper receiver and don't touch the barrel at all, "floating" out there. FYI, they also make gas blocks that are railed on top so you can attach stuff to them....personally, just put a low profile gas block on, then get a free float of the length and flavor that you want and attach the front sight to that.
About the gas tube, Do I really need it or is there another way?
Yes. 223 semi-automatic AR-15 pattern guns operate on the principle of bleeding off gas from powder combustion to operate the action. Without the "fuel" the engine won't run.
On the other hand, if you wanted a 9mm AR carbine, they operate purely on blowback...ie. no gas system.
What I would do is start looking at pics of ARs and looking at parts lists to figure out what is what. Then you can figure out what you like and what you don't. You could even find a pic of one you like, then shop around and build it similar.
What you need to really understand is that barrel length, handguard length and gas system length are three independent entities.
See this graphic:
Ok....mix and match. Choose the barrel length you want, the gas system length and the handguard length.