FWIW - I have had the same TD-166 MkII since I bought it new in 1984 (when I feared that turntables would no longer be available - hah!). I have had it modded somewhat - some damping of the platter, a new mat, new RCA cables and new feet to better level the 'table. Also added a KAB record clamp, which is a bargain and perfect for suspended 'tables that can't use heavy clamps.
All I can say is that I have brought some needle-drops made on this table (with either an Ortofon OM-30 or Denon DL-160 cartridge) to some pretty high resolution systems, and have received a lot of completments on the sound quality.
As for the age, these older, West German made Thorens' are built like tanks. All I ever did to it, maintenance-wise, is a new belt.
I once was at a Stereophile show, and at an "ask the editors" session on analog audio, asked the panel how much I'd have to spend to improve on the TD-166MkII on a modern 'table. Art Dudley, of course, suggested I stick with my Thorens. Others suggested something in the $2000 range, which considering inflation, is not much more than the Thorens cost me in 1984.
I guess my advise would be: If you can get a good deal on a well-maintained and perfectly operating vintage 'table, go for it. But to spend a lot on a fancy plinth or upgraded 'arm, I am not so sure. Perhaps once you get into the $2,500 range, you might look at something like a VPI Scout. Just my $0.02.