I don't believe catnip plants will have any big effect unless they know to break the plant, as its the oil released from crushing it that attracts them if I'm not mistaken. Then it wouldn't be low maintenance though, as a plant can't be healthy and grow if its being chomped and romped on all the time.
So unconcentrated living catnip may be "fresh" but I bet you it'd be far less effective than buying some processed concentrated stuff online.
Really cats don't need plants, that hamster food is likely unhealthy and should be kept away, and eating too much of that wheat grass can induce vomiting. I tried it, but it seemed expensive with no real benefit I could find evidence of online. They liked it, but I just got them toys instead.
For example you could get little springs like these:
http://www.amazon.com/Colorful-Springs-Cat-Toy-Wide/dp/B000CMKHDG/ref=sr_1_1?ie=UTF8&qid=1322804357&sr=8-1
And your kitty would likely spend just as much time munching on it, but it never goes bad, and no concerns of ingesting too much and puking.
If you do want to get plants, I'd just make sure they aren't on the no-no toxic list, but do your best to keep it out of reach else they may make a mess and/or destroy it. If its purely for aesthetic value, make sure there are no small parts they'd want to eat, and just get an artificial one IMO. 