$3500 floor standing speakers used. What would be your choice ?

excellent choices. I have a few speakers and I would narrow down to these 2- both American (I am not).

Omega- single driver speakers. I have the Junior 8 XRS. Even new they would be within your budget. For me they are almost the best speakers I have had, besting even stuff more than 2x their price.

Devore Fidelity Orangutan Baby. Used they could just be within your budget. Again amazing sound.

A third one- which might not come within your budget, used are from Poland. Qualio Audio IQ.

Rosso Fiorentino Elba 2. 

Used Devore Fidelity Gibbon 88 US audiomart has a pair $2450.00

A lot of advice provided in this thread...

Because you have a really nice, clean and simple system - Nottingham into your VAC integrated (love that amp, and I am vinyl based also). I think match the integrated to the speakers (think 93bd and up), and the speakers need to match the room...

But my experience says go for something like the Focal 1038. It is as good as the Sopra's 2 or 3 IMHE, with bass more like the 3. Easier to drive with tubes than any Sopra (I have heard them just Rock out (loud) to large Brahms or Miles KOB on as little as a 40w AudioPrism EL34 amp in a 12x27 room). The 1038 is a large speaker system, but small enough for a good match with your integrated, will fit a lot of rooms. 

This is quite a range of very different speakers. From British ProAc, Tannoy and Fyne to Italian Sonus Faber and Rosso to American Devore, Silverline, Varndersteen and others to French JM Labs.

How do you choose if buying used and can't really audition most of them?

And again, no one describes the sound of their recommended speakers in details.

Like mulveling, I prefer simpler speakers with as few drivers as possible. And that's what I currently have - 1" silk dome tweeter and 8" woofer, both made by Vifa.