I’ve dipped my toe into these waters and the two biggest factors (for me) are original and complete. Condition is not irrelevant but it’s much easier to find replacement stuff if you know what you’re looking for. An original derailleur on a 1950s English bike (for example) that is rusted solid gives you an idea what to look for. One of my bikes is missing the brakes and originals are some bizarre cantilever-ish set up that seems impossible to find.

If you enjoy a challenge, this stuff is a fun learning experience. If you’re eager to get the project done so you can ride it, substitute stuff can be used but where is the fun in that!?