Of course, as with antiques or any form of collecting, the main point of collecting vinyl is to enjoy it - that's why I say you can pick up ANY Red/Plum Zep album and it will sound amazing, if it's not scratched to death.
There are certain presses that sound better than others though - and there are rumours I've heard about some early pressings being mastered in such a way that cheaper record players simply couldn't handle them, and many "scratched" copies of mind-blowing vinyl were returned to stores - so a really early press may not suit your system if you're on a tight budget.
For example, some US audiophile collectors swear by a particular US pressing of LZ II with the initials RL in the dead wax - apparently copies can still be found in secondhand shops for less than $10. The sound on this pressing (RL was the mastering engineer) is supposed to render the music breathtakingly lifelike (and is the one I heard the above rumour about) - but I couldn't verify that, as US presses hardly ever turn up in the UK.
Like "Antiques Roadshow" (UK TV show), there's always interest in what it's worth - that's another part of the fun of collecting - but I mainly want to find the finest possible sounding recordings of the music I really dig.
LZ III is a superb album. I just double checked mine, which I got from a local shop for £10, and yes, it's definitely the earliest press possible, and Near Mint - which is nice.
Now I need another that I can actually play without worrying about decreasing its value...