Could slightly older models effectively have substantial differences with the newer ones?
However, I'm concerned about some features they may not include; I'm not talking about pro-level features such as IR transmission (which is something that I don't need right now) but maybe compatibility with flash transmitters, proper TTL, recharge time, focal length synchronization, and so on.
Ironically, the 580EX and 430EX models you listed can be used as IR slaves in Canon's optical TTL system, they all do TTL, have pretty much the same recycle times as their successors, and zoom (although they might not zoom quite as much. On camera, they're pretty much as capable as the later models. An EX designation means the flash does TTL on a Canon dSLR, and can be used as a wireless slave.
The E and EZ models are from the film era (so do aTTL, not eTTL) so can't be used in TTL on a digital body. They also can't do the IR wireless slave thing (only EX models can). And some of the older models don't have M mode (the 540EZ does, but the 420EX doesn't). If you have a TTL-only flash and you are using radio triggers that cannot communicate TTL and are "manual-only", there's no way to adjust the power level on the flash. M mode gives you this capability.
But the compatibility with off-camera flash triggering is the main reason you may want to avoid anything older than a 580EXII or 430EXII. There are reasons earlier models are cheap and plentiful on the used market.
Prior to the Digic 4 processor, Canon cameras and flashes only talked to each other through the hotshoe. Canon dSLRs didn't have a flash control menu in them, and you did everything on the back of the flash. With the advent of the Digic 4 processor and the Mk II EX models, you could now control the flash and its settings from the camera. A lot of third-party TTL triggers, such as the Yongnuo YN-622, seem to be leveraging this capability to do remote flash control over TTL radio triggers.
I owned a 580EX and 430EX, and got YN-622 triggers, and found that I could not control M power levels (1, 1/2, 1/4, 1/8, etc.) but had to use TTL ratios, as with an ST-E2. I did have HSS. I had no access to zoom control, group control, or wireless settings. I had no access to custom functions. I had no access to most of what I bought the triggers for. For that, I needed a 580EXII or 430EXII or later flash.
If you plan to use this flash primarily on-camera or both on-camera and off, then I'd still recommend looking into getting a used Canon flash.
But if you plan to use this flash primarily or only off-camera with radio triggers, then a better plan might be to explore some of the 3rd-party offerings with built-in radio triggers. Godox, Phottix, Nissin, Yongnuo--there are a lot of 3rd party options aside from Canon these days, and most of them cost a lot less than a new or even a used Canon flash.
See also: the so-called "EOS Flash Bible" if you really want to get into vintage Canon flash gear.