I’ve been working on “retrofication” of a “somewhat” modern case and have some notes for that process more than a specific recomendation:

• If you want a 3.5" floppy that works, you can either get a little USB adapter board, or a LS-120 on a PATA-SATA adapter. A Caleb UHD-144 might work, but many BIOSes and Windows still special-case LS-120s as “it can be drive A.” The USB adapters sort of suck, because the USB floppy spec sucks. 5.25, the best you can really do would be to rig up a Greaseweazle (specialised USB controller) which won’t really work like a regular floppy drive

• Instead of a MHz display, you can get a small programmable OLED. Digole offers some that can be hooked to a $1 USB-UART adapter and programmed very easily-- I’ve got some crude code hacked up (C++ for Linux) on my Gitlab (https://gitlab.com/hakfoo1/graphic-oled-control-for-linux) that gives you MHz and a bunch of other stats, but you could probably also rig up something on Windows

• Rustoleum Heirloom White is a very good “panel beige” for the metal side panels. You might also look at their “Satin Ivory” which gives a slightly yellowed tint good for the front panel.

• Intentional colour mismatches can work very well, like if you use an optical drive, paint it a different beige than the rest of the case, to indicate either being different plastic than the main case, or an aftermarket add-on. Tell a story.

• Some features feel like they “post-date” a case. Top-mounted ports seem pretty uncommon on vintage cases the first one I can recall having was well into the Windows Vista era)

• Get a momentary paddle switch, preferrably red, and mount that and it buys you a mountain of street cred.

• Grilles can be cut away and replaced with 3-D printed alternatives.

Not quite as retro, but I recently built a gaming HTPC in a Silverstone gd09 case, and my friend described it as having strong “2003 [middle-to-upper-middle-class suburban town we grew up near] basement” vibes.

For reference, we’re from the next town over that was historically kind of a working class mill town, and we’re from townie families whose parents and grandparents worked in the mills and factories once upon a time. Same school district as that other town though so lots of friends from there. Circa 2003 we were probably hanging out in their finished basement rec rooms with a giant rear projection TV and watching stupid flash cartoons on the family desktop.

There isn’t much. Not that I can find easily, anyway. Depending on your actual hardware, you might be able to get away with just using an old case. mATX has been around since the late '90s. Only thing I’d watch out for is ventilation.