1) The midlength gas system really won't keep things cleaner, but on a 16" barrel it feels a touch smoother than the carbine length system. Reason is, the carbine length system was devised for a 14.5" barrel. On a longer barrel you end up with more "dwell time" (the time period that begins with the bullet passing the gas port and ends with the bullet exiting the muzzle; it's essentially the time in which the gas system is getting full pressure). That means gas pressures are higher in the system, and that ends up with the thing cycling a bit rougher. A carbine length system will work just fine, but the preference on a 16" barrel is for a midlength.
2) The fixed front sight won't bother you if you add an EOTech or Aimpoint on down the line. Seriously, it won't. You will be focusing on the reticle/dot and not the front sight post, and if you go for a lower 1/3 cowitness (the iron sights cowitness through your optic at the lower third point of the field of view) mount, it really won't be any thing. IMO, unless you just absolutely hate the idea of a fixed front sight base or just want a really long rail there's really no reason to spend more for the low profile gas block and mount a backup to the rail system.
3) Beats me. You'd probably be doing decently to get it for under $1500, but that depends on whether the gun shops in your area tend to be high or low priced.