Hi @Henny22
That’s some piece of kit you’ve found there! I think it’s actually older than me (just)!
If you’re only looking to get audio from it into Sonos, then a cable connecting the headphone output to a Sonos line-in may be the easiest (as in, you might already own the cable), but you would need to adjust the output volume to get the best signal to noise ratio - it may take some experimentation.
Using a 5 pin DIN cable would perhaps be a better option (no need to adjust the output volume) but there may be complications. I recommend a read of https://en.wikipedia.org/wiki/DIN_connector and careful selection of a 5 pin DIN-RCA or 5 pin DIN-3.5mm jack cable. If you’re only looking to output from the Soundeck, it should be simple enough.
Be warned, however - that system has 3 sources, and the only one not subject to wow & flutter is the analogue radio. Most people seem to have forgotten all about wow & flutter, presumably putting the trauma behind them. It’s when the speed of a tape (or record) varies slightly, which produces small variations in pitch. The tape deck is likely to be the worst performer in this regard. Analogue radio may not make it past 2030 in the UK - it’s shutdown has already been delayed, and the powers that be may not be willing to extend it’s lifetime beyond that.
It won’t make any difference which of the mentioned Sonos devices you use - you’ll just need to change which cable you get. Either option would allow you to play on all your existing speakers, though of course a Play:5/Five is a speaker in it’s own right. If you buy second-hand, be sure to get something that is S2-compatible.
I hope this helps.