As others mentioned, a KZ400 is a great first bike. Parts are getting a little scarce, though, so you might want to consider a KZ440. I bought a KZ400 for my girlfriend to learn on, after starting on a KZ440 myself. In retrospect, I'm glad I had the experience working on the 440 before I tackled the 400. The 400 was a non-runner when I bought it, & I had to improvise a bit, especially with the carbs. I don't know that I would have had the confidence to do that if not for the experience with the 440.
The more modern EX250 & EX500, AKA Ninja 250 & Ninja 500, are also excellent starter bikes, though I wouldn't want to steer you away from the KZs & they don't have that classic look.
Good luck with whatever you get! If it's a KZ, this is the place to find knowledge. I really feel like the people here taught me to be a mechanic! If you end up with a KZ400 or KZ440, it wouldn't hurt to check out Odd Ivar's forum as well,
www.armbell.com/forum/index.php?mforum=kz400
. He was very helpful to me, & I don't know if he posts here anymore. This forum has great people on it, but there's more knowledge here about the bigger 4-bangers than the little twins. (Not to say that you won't get good help here, & a lot of the same stuff applies!)
HTH,
-Kit