What assembly is there in an upper? All I can think of is attaching the barrel/chamber to the receiver. And the gas tube. I've always just accepted that as 'gunsmith work' and kind of treated upper receivers and barrels as a matched 'I really have no reason to ever take this apart' pair.
ejection port cover, forward assist, barrel, gas block, gas tube, and possibly the BCG/Charging handle depending if you buy parts or already assembled on those.
Tool wise for the upper you need a barrel nut wrench (most armors wrenches do this and the buffer tube), a method to hold the upper (usually upper vice block), and preferably a torque wrench.
The forward assist and ejection port cover take all of 5 minutes to do and are easy. BCG's and charging handles arent hard at all. The barrel is pretty simple you just need the tools. You want to torque the barrel nut down to 30ft lb- 80ft lb...or more whatever number between those gets the holes to line up with the gas tube. Gas block is simple to install you just have to make sure the holes line up. Same with the gas tube.
The rail you pick sort of depends too. For instance my troy alpha rail uses a standard barrel nut. But some other brands of rails use their own barrel nuts.
As for it being "gunsmithing" work...its very basic gunsmithing work. As long as you buy a quality barrel and not one made in redneck joes basement they will be headspaced correctly and good to go right out of the box.