i think you might overestimate the building part.
If you get a barebones board with a hotwap PCB you have to screw in stabilizers for all keys larger than 2u (=2 keys) - Shift, Enter, Backspace and Space. Since you can't be arsed, screw lubing them. Just screw them in, takes 3 minutes.
Then, place all the switches on Plate and PCB and since they're hotswap you just press them in.
Then place like 8 gaskets or screws, drop on your PCB-Plate-Switch-Sandwitch, screw together top and bottom with 4 more screws.
When you're done, mount the caps, also just pop them in place.
That's it. I reckon it's about as much work as a 10$ Lego set.
But going prebuilt is far easier of course: Unpack, plug in, done. If you get a hotswap you can change switches later on if you fancy.
Drop is one of the rare shops that don't run group buys, but have in stock items instead. Mostly. No need for fomo unless stated otherwise on the product page.
Also, they have at least one buy-one-get-one-free sale per year. Because of that, you have a good chance of getting one second hand for less than a new one.
Regarding kitting you have to watch out for modifiers in the correct size for the correct row, but most keyboards are fairly standard with the exception being the bottom row. Check Shift, Ctrl, Alt, Space and Fn/Win for correct size.