dammit. i just spent like 5 mins writing a response to all your questions and browser somehow jumped back to the main page....so here it is in short
Google the o2 sensor for the supra. or visit stores you know. you can get it for under $100 easily. I heard Bosch isnt that great, people say they crap out pretty easily. Although that is just speculation. The Denso sensors can be different. you can get ones with the connectors, or with just the wire to connect to the old sensor wire. and here is a link on how to heat shrink all that stuff.
http://photos.dylanwiggins.com/thumbnails.php?album=146
as for your fuel line, do you ever use fuel injector cleaner? it does wonders if you use it. I make it a habit to use it every 1500kms or so. just to keep my fuel system good.