Well, on the topic of bikes to stay away from, to paraphrase Sheldon, the dimensions and thread standards used on older French (Motobecane, Peugeot) and English (Raleigh) bicycles can make it difficult to find suitable parts for repair or upgrading if you're trying to keep the conversion cheap.

Also, stay away from anything with ashtabula cranks as they can sometimes make obtaining a proper chainline very difficult (besides being heavy and crappy and usually indicative of a heavy and crappy bike all-around). Also, avoid any frame made out of high-tensile steel (Schwinn Varsity comes to mind): again, heavy and crappy. Your frame should be constructed from some type of CrMo steel tubing at least.

As far as detailed catalog specs go on older models, you might have to do some creative googling to turn up some results. As far as older Treks go, http://www.vintage-trek.com is a good resource.