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

Harbeth 30.2 XD 

Read TAS review. I have them, they are great imo.

Iʻd put together a SEAS Thor kit from Madisound ($2615.00).  They have a number of cabinet makers who can build them and ship them to your door for a reasonable amount.  Theyʻre not all that hard to build yourself.  You could get help from a friend, or just have a local cabinet maker do them up.  

The SEAS Thor kit uses the same tweeter and very similar SEAS Excel drivers to those used by the $14,000 Joseph Audio Perspective loudspeaker.  Thor is a transmission line and will give you a good low end.

Try finding some JBL on Craiglist you won't be sorry

You could buy a new pair of Monitor Audio Silver 500 or 300 floor standing speakers with that budget.

You have some good options (some already mentioned - ATC’s, Magnepan, Vandersteen, etc.) - but - my choice would be (was) - the following:

1. Linkwitz Orions or LX521s - nothing compares to Linkwitz open baffle speakers (is any better). You can find used Orions for <$5,000 - sometimes for <$4000. LX521’s sometimes for <$5000. And... that’s with the amplifier, the ASP and cables. Believe me - once you’ve heard these in good room - you will find nothing better - and - your search will end for the rest of your life. As noted below: "Linkwitz speakers are known for their realism and 3D sound."

2. Magnepan LRS+ - which comes close to the Linkwitz speakers - again - very few others compare. You can get these new for $1000. And with the right equipment almost nothing is better. The great benefit of these is - that they’re very light weight and are easy to move about to optimize their sound in your room, as well as move out of the way, when not in use. Yet - you still have "World-class" sound... and... you can get a pair for "pocket-change."

3. And... if you’re willing to compromise with a superb stand-mount speaker - almost nothing compares to the Legacy Audio Calibres - which sounds as good as most floor standing speakers and is much easier to manage. Though these are a bit more expensive (around $6500 new).

Got all of these and more - and they are all superb. If you’re an acoustic music fan (jazz, classical, blues, country, blue grass, etc. - real music) all of these are great! If you’re into other sounds - the Linkwitz, or Legacy speakers might suit you better - with a wee bit better bass and dynamics than the Maggies.

Of the others - my choice would prolly be the Vandersteens, because - like the Maggies they’re virtually an open baffle, dipole speaker - which is superb with "all things acoustic."

Because all of these are open baffle, or hybrid open baffle, speakers - as noted - virtually nothing else compares. Far superior soundstage, image, and "life-like" sound.  Very much like their ancestor - the Quad ESL 57's - which is the "king of the hill," since the 50's/60's.  

But, you will find nothing better for <$10,000+++).

Some educational links - say it all:

"Vandersteen Audio and Linkwitz Lab are both companies that design and make loudspeakers. Vandersteen speakers are known for their natural sound and dimensional purity, while Linkwitz speakers are known for their realism and 3D sound."

https://www.google.com/search?q=Vandersteen+linkwitz+audio+lab&oq=Vandersteen+linkwitz+audio+lab&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIHCAEQIRigATIHCAIQIRigATIHCAMQIRiPAtIBCDk1MzJqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8

https://www.linkwitzlab.com/