First - Welcome to the forum!
Does that machine have a graphic display? A display is not required to learn this hobby, but it helps. Don't know about that machine - never seen one.
You might go to the White's site to see what their list price is for that unit. (If you don't see that machine there, then go to the Kellyco Metal Detectors site and look for it there - White's made certain machines for sale by Kellyco only.) As far as a 'used' price goes, consider this; you can get a new Garrett ACE 250 delivered to you for about $217. That includes a guarantee and an instructional video. The 250 is a very popular machine and if you decide you don't like this hobby, you could turn-around and sell it on ebay for $170 or more (as long as you didn't beat it up and stomp on it - keep the box.) It costs about as much as the Prizm II, but has a lot more bells and whistles for the money.
If the person selling the Classic III is a friend of yours and is willing to teach you how to use it, then that's a plus.
As far as the Prizm line goes, I had a Prizm V and couldn't sell it fast enough. Just didn't perform the way I like my machines to perform. I'd even point you toward a Bounty Hunter before I'd recommend a Prizm - but that's just a personal thing. Lots of folks use Prizms and love them.
I hope you enjoy the hobby. Don't get too hung-up on which machine to get. Stay with White's, Garrett, Fisher, Tesoro, Minelab, and Bounty Hunter brands and you'll end-up with a good machine. The first few times out could be confusing; first search your back yard, then find a 'tot-lot' and search that. Don't go to an old picnic area first - they're full of pull-tabs and bottle caps and you'll dig 50 bottle caps for every coin you find - a very frustrating place to try and learn how to use a new detector.
HH
Rich