Vintage Thorens turntable -Good or Bad?

Can't speak to the 126 but I bought a 166mk II and it plays and sounds great. It is in excellent condition and shows little sign of wear or usage. I did put a new belt on it and bought a cartridge for it. As to whether yours is worth it or not I guess it depends on how much you paid for it and what condition it is in.

How much you paid for it certainly has everything to do with the answer to your question. Unless you paid too much, or something is wrong with it, you are not wasting your money. The TD126 was a fine sounding table in its day, and you would probably have to pay a lot more than you have in it for better today. It is certainly worthy of a decent cartridge.
A high-end shop may not be interested in an old Thorens, but I think you might be surprised how good it sounds. If the bearing never dried out, and everything works, it may be worth upgrading. Tonearm wire/cables, maybe tonearm/armboard.

It's a good deck.

If it has the stock Thorens tonearm (TP16 MkII) it requires a high compliance/compliant cartridge, due to the lower mass (<8 grams I think).

When setting it up look into redamping the springs as the foam sponges (located inside the spring coils) break down with time.

I used wedge shaped makeup sponges in my TD125, but research what others are using for replacement parts.

Maybe someone was having a bad day @ Brooks?

I would rate the condition of this deck very clean 8.5 out of 10.. maybe better when it's cleaned up Everything is working on the deck. Stock TP16MKII tonearm with a Stanton 680 Cartridge- stylus bit is worn..

I have owned a TD-166MkII since I bought it new in 1984. It has always sounded fine to me, and has been very reliable. I bought it when i thought turntables would become extinct, and I refused to re-purchase all of my vinyl in CD form. Who knew vinyl would have such staying power? Anyway, I wouldn't classify any Thorens as a "lemon." They are very well made and designed. The TD-126 MkII is no exception. I have the stock arm with two wands. One has an Ortofon OM-30 Super MM cartridge, and the other a Denon DL-160 HOMC cartridge. They both sound great and track beautifully, with the Denon sounding a bit smoother and richer in my system. Years ago, I had a Grado in it. Not sure why I switched it out, it was a long time ago.

There are many tweaks and mods for vintage Thorens 'tables. I just had a local modder, K-Works in Clifton, New Jersey, do some work on my TD-166MkII. I had new feet installed that provide better damping and leveling ability, added a lot of vibration damping to the plinth and platter, added an Ersa-Mat (I think TTVJ or Needle Doctor sells this item), and upgraded the RCA cables. Too early for a full evaluation, but improvements seem to be subtle, but noticeable, including better bass definition and perhaps a smoother presence range.

Don't let what the shop guy said bother you. Either he wants to sell you a new 'table or he doesn't know much about Thorens decks. Do a web search. There are many sites devoted to vintage Thorens 'tables. Enjoy!