I've got a 1978 KZ650 (Kawasaki). If that bike was in running shape I'd say go for it, but like others have said - you may end up putting in twice what you paid for it to get it running perfectly. I bought mine for $900 in running shape but it needed some work. I've probably put at least $500 into it since then and its getting there. I'd say get a running bike, it doesn't have to be running perfectly but you need more than one gear. These old Japanese bikes are great but when you're looking at a bike that's over 30 years old, you're almost guaranteed that it will need some work. Get one that's running - you'll be able to at least get a feel for motorcycles and will still have plenty to tinker with.
I'll probably get a new bike eventually, but I would not hesitate to take the old girl across the country - and I don't plan on ever selling it.
I'm going to follow up and say that I would not recommend getting a rebel unless you find an amazing deal. It would be good if you have never ridden a motorcycle before, but you will want something else within a month or two unless you only plan on cruising around town. They are just so tiny and silly looking...