Quoted:
I've read the "Start Here" posts and some Noob posts but I'm just getting more confused.  I understand buying the parts and assembling them to get the exact weapon you want or to save money or just because you enjoy that sort of thing (which probably is the biggest interest for me).  But I don't understand things like what I can swap out to have a different look or functioning rifle.
Like:
Swap a flat top for the handle top? Can be done, requires tools and a little skill, only part difference is the upper receiver, may not be worth the trouble, buy the one you want initially, recommend buy a flat top and a removable carry handle.  If you like the carry handle you can leave it, if you want the flat top it's easy to remove the handle or even to swap back and forth

Swap a solid stock for a tactical stock? Can be done, requires tools and a little skill, None of the stock parts are interchangeable between fixed and collapsible, Depends on what your needs are, kind of a pain to swap back and forth, recommend you decide which style you want before you buy

Switch between a rifle style and a pistol style?  Can't be done legally,  once a rifle always a rifle.  You can make a rifle from a pistol but not the other way around, and there's no going back once you put a stock on, do not put a stock and a barrel shorter than 16" on the same gun

Swap a .223 barrel and a 6.8 mm barrel to change calibers?  Or do I need to change the upper and lower too?  Would it be easier to just build two separate guns?  Can be done, requires tools and a little skill, only part difference is the barrel and bolt plus magazine, may not be worth the trouble to go back and forth (too much wear on the upper receiver threads), recommend two complete upper receiver assemblies that can be snapped onto a single lower receiver assembly

I like the 6.8 mm from RRA and I like that big .450 BushmasterAgain, having complete upper receiver assemblies allows you to swap out calibers quickly and easily, the only exception would be a 9mm which requires a magazine block to be inserted into the lower (unless you have the Olympic Arms stock magazines).  With respect to 6.8, I recommend you research the various twist rates and chambers and think about what ammo you want to shoot out of it before you buy

What parts would I buy first?  Since the Lower Receiver requires buying from an FFL that would be first on my list.  Everything else can be obtained mail order.  You can buy a lower and then make final decisions on what type, style, caliber, etc. rifle you want.