I am about to get a Thorens 160 in excellent condition in trade for some of my unused recording studio gear and I would like to get an older integrated of at least 100 watts 8ohm to go with it. I would like to stay away from brand new because I would like something with longevity and will be worth roughly what I paid when and if I decide to sellI think your looking at this the wrong way.
An amps primary purpose is to properly drive a pair of speakers. All other functions are just features that can added. So find the best 100 wpc integrated (with or without a phono stage) that will drive your speakers and then add an external phono preamp to match your cartridge if you have to. Besides, an external generally will outperform the built-ins found in most integrateds/receivers, new or vintage.
Also, what makes you think you can't get longevity out of a modern integrated and that it won't hold its resale value too? You just need to research them and know where to look because you won't find them at a big box store. The Audiogon classifieds is a good a good place to start. A good, modern 100 wpc continuously rated integrated will run rings around the best vintage integrated as the modern most likely will be 4 ohm rated. Very few vintage amps were because all the speakers were 8 ohms.
Concentrate on getting the best sound possible from your speakers and TT. Not necessarily matching vintage with vintage.