I prefer old steel over aluminum. I think you are somewhat limiting yourself by focusing on Shimano 105 or 600. SunTour's high end components were also very good in the mid 80s.
I think your friend is wrong about how much you should spend on older bikes. If you are spending about $250, then a mid-80s bike is a very good value compared with anything else you can buy in that price range. Local markets vary a lot so it is hard for most people elsewhere to comment specifically about what price is reasonable in your market. If you are spending hours every day looking at CL and ebay, then you should know your local market much better than your friend or anyone else who hasn't spent the time looking at your specific local used bike market.
In terms of which bike to choose, the centurion and the Trek 560 would both be awesome bikes. If they are in great condition, there's no way you should expect to get bikes like that for under $200. The one that fits you best and feels most comfortable is the one you should get. Many sellers do not feel comfortable letting a stranger take their bike for a long test ride, but that is what you really need to do. When I'm selling a decent used bike and I have a serious buyer, I suggest we go together on a 10 minute ride, with me on another of my bikes. Maybe you could ask the seller if that would be possible.