1. not sure. What year are they?
2. his speakers have always been so neutral and the threes will move more air than the Treo's, just don't expect to get the same tightness etc... Compared to much of the stuff out there now days, I personally feel the soundstage is as good or bigger and that's on the older ones.
3. As far as separation, 8' is fine. Just go to the site and make sure you set them up properly with the proper tilt back. They will sound like a point source and are time and phase correct as you know, so they won't smear. That will give you the layering of any genre you want.
The newer the better IMHO. They will make changes and not change the nomenclature of the speaker, so a 20 year old 3 isn't your best buy. I'd much rather have a newer 2 that has the newer drivers that filtered down from the original Quatro's. THAT would be a good deal. If it's not that, personally, I'd try to save for a pair of new 2's, but that's still more. I'm sure that you will get a good deal within your budget and get a great speaker. If you know the serial numbers, sometimes they can help you on their site in the ask Richard area.
Please keep up posted.