I wouldn't bother with much of anything unless the bike looks clean and lightly used... you're obviously not an experienced mechanic and you could wind up just buying a money pit, paying $20 for a bike and then putting $$$ into it just to get it rideable. The biggest key with a roadbike is fit, so if you buy one that doesn't fit, you aren't going to like roadriding... so make sure you fully understand proper roadbike fit.
If stuff like the handlebars are rusty, thats a sign that its an old POS and probably not worth your time, since good quality bikes, even back 20 years ago or more, typically had aluminum/alloy bars and components vs. the steel which rusts easily.