So I'm trying to piece together my .308 build but everyone seems to be in full panic buying mode and is snatching up everything i've had on my list like their hot cakes.

I was planning to do an 18" barrel but the one i had picked out is now sold out. I'm now wondering if I should just jump on the 16" barrel of the same maker/twist and such.

Would there be a noticeable difference in the performance of my rifle at medium to longer ranges with the shorter barrel or am i likely to not notice it unless i'm really stretching it out? (which I like to fantasize about but the reality of it actually happening is slim and rare)

without knowing what exactly you want to do with this rifle, its hard to answer for you.  will a 16 inch work fine, yup.  will a 14.5" barrel work fine, yup.  so will 18-20-22 inch barrels.  if ur not going to shoot much past 2-300 yards, 16 is fine.  if ur looking for sub moa, a longer barrel wouldnt hurt to have.

it'll be for mainly 300 and under but i'd still like it to be MOA accurate out to around 600 ish.

It'll see alot of range time but i'll also try and take it out on hikes or hunts as much as possible so keeping it light isn't a bad idea. I'm just curious as to how much of a difference that 2" would make when I do get the rare chance to shoot out past 300 yards.

I've read the .308 gains very little for barrel lengths over 20 inches.  Some loads (I read) do not lose all that much going down to 16.

A friend has a DPMS Oracle with a 16-inch barrel.  Mine has a 20-inch barrel.  He has not shot for groups recently, but at sight-in was in the MOA range.  I'm at 1/2 MOA with my best loads.

Mine outweighs his by 5 pounds.

If you go long, there are benefits, but remember the weight!