I just picked up a 1980s Nishiki Cresta for $150 on Craigslist. Rode it 20 miles today. It was hardly used.
I think it would be a great experience to start with some of the older steel frames from the 70s or 80s. Just make sure you don't have to put money into it from the get-go and you can keep the price down. Definitely get the experience of riding steel, though, so you'll be able to compare rides to aluminum and carbon. Who knows, you might find you like the bikes and live the adage - Steel is Real!