I'm new to cycling but, as with many of my other hobbies in the past, I want to tinker!

My (only) bike is a new Trek 7100 Hybrid and I figure it's better left alone, unless you all want to suggest some upgrades worthy of this cheaper bike.

What I would like like to do is buy an older bike and clean it up, repair and upgrade. I have read a lot about steel frames and that seems like the way to go on an older bike. What specific brand do you all suggest? what type of steel and how do I find out if that's the steel the bike is made of? Lastly, I would like something old enough that components can be found reasonably priced on close-out or on ebay and I would like suggestions on specific bikes suitable for that or ones to stay away from. Don't forget, this will be my first tinkering project and I don't want to have real expensive mistakes.

Thanks to all,

Mirko