In your shoes, I’d put the money in a proper case (eg. fractal node 304/804) rather than an USB enclosure (no, you don’t need hot-swap for a home server): besides the performance issues of USB (which may or may not be an actual issue depending on what you plan to do with the NAS), having a single box makes everything simpler.

For components to fill up the case, you can look at second-hand computers on ebay.

As for the OS, if you are not familiar with linux you may want to look at truenas scale (which is linux).

If you never built a PC, you’ll have to do a lot of research not to buy incompatible components… otherwise you could rely on a friend/shop or stick to sinology and similar.