1. Most folks who buy 14.5" barrels have an extended-length muzzle device permanently attached, so that the muzzle device counts towards the overall legal length of the barrel.  For folks with a pre-ban rifle, a 14.5" barrel allows you to have a working flash suppressor such as a Phantom, Vortex, or other extended flash suppressor and still keep the over-all length at just over 16", avoiding NFA issues.  The standard A2 flash suppressor is too short to get the barrel out past 16".

For a post-ban barrel, my preference is to use a 16" barrel, and have a fake flash suppressor press-fit over the end of the barrel.  Since you can't legally have a flash suppressor, and you don't need a brake, this setup gives you the max velocity, and short over-all length while still maintaining the look of a 14.5" barrel with a FS.

2. I am assuming you meant "gas tube", not buffer tube.  The gas tube and gas port location are in the same location for 14.5" and 16" barrels.  A 16" barrel just has more barrel in front of the front sight.  There are no issues here; military M4s use the 14.5" barrel.

3. There could be many issues to consider.  Is your rifle pre- or post-ban?  What do you plan to do with your rifle?  What ammo do you plan to use?  Will this ever be a defensive weapon (should the need arise), or will it be strictly a range toy?  Are you looking to mount optics, and if so, which one(s)?  Are you planning to install railed handguards, and if so, which one?

-Troy

Given that you're going for a pre-ban upper, and you aren't going to need to remove the front sight base or barrel nut (neither is necessary to mount a RAS), then I would recommend a 14.5" barrel with a permanently-mounted Phantom.  Total length is 16.1" or so, you get a great flash suppressor that won't break on you (Vortexs have had issues in the past), and it preserves the military look and feel.  Heck, you can even mount a bayonet properly.

-Troy