Unless you're dealing with very obscure hardware, finding drivers isn't such a problem. If you stick to more typical hardware, finding drivers and applications is really no different than working with more modern systems. However, it sounds like you want a more turnkey/spoonfed solution - depending on the games you want to play, DOSbox may be a much friendlier alternative (and this is partly why I suggest you identify what games you actually want to run, especially as it sounds like hardware isn't what you're after).
As far as old parts working or not working - buying a complete system is no guarantee, nor is buying hardware that is listed as working/tested. Most of the parts we're talking about here are approaching twenty years old, and a lot of this stuff was not designed with a multi-decade lifecycle in mind. Stuff breaks. IME its usually better to go piece-by-piece, as you have more granular control, and usually better knowledge of the condition the piece is coming in before-hand. A lot of complete systems online rarely have internal pictures in the listings, and a lot of shops don't like you popping computers open to "take a look" either. By contrast, individual parts usually give you a pretty good picture of what you're looking at, so you can at least screen (to some extent) for things like broken connectors, bad caps, leaking batteries, damaged traces, etc.
