I agree with learning on junk, but the rest is really bad advice. Don't go for maximum challenge right out of the gate. This leads many to the proverbial five-gallon bucket full of bike parts and the proverbial ball bearings all over the garage floor. I would suggest starting with a fairly tuned up bike, riding it, and learning how to deal with each issue as it arises - one problem at a time. It is also genuinely helpful to have another brake or wheel or brifter or whatever to look at while you have the other one in pieces.