First order of business is to determine if you can correctly place and power a pair of SL3's. If you read up a bit on this forum, you quickly learn that getting good results with the planar dipole speakers is all about how they are set up in the room, which usually involves the speaker panel being 4' or more away from the wall behind it. In most 'living room' setups, this is a hard sell to the spouse.
Next up is power, electrostats need a good power amp to let them do their thing, most receivers do not qualitfy. You'll need a good power amp capable of doubling its output with every halving of impedance (e.g. 200w @8 Ohms, 400w @4 Ohms, 800w @2 Ohms). It needs to be stable down to 3 Ohms or so.
If you can get past those hurdles, the condition of the SL3 panel is the most critical, as replacing those will likely cost more the the purchase price. You want the panels to be clean, no obvious sign of deposits on the transparent mylar. Older units will sometimes have little dots of darker shading where dust has bonded to the panel. If it's just a bit, not to big a deal, if you can't see through it, then something to worry about.
Panels can be cleaned, but results are mixed.
Check the woofer using a strong flashlight to see the driver, looking for issues with the surround. These are older drivers and may or may not have been abused. If the system they were in had a sub, then better, as they might not have been stressed as much.
Be aware that even if the panels are OK, they probably will need replacing in the next 5 to 10 years. They will still work, but will decline in performance. That said, I have a pair of Sequell IIb (the predecessor to the SL3, with same exact panel as the SL3) whose panels are 25 years old and still performing well enough as my rear speakers. But then, I baby them, and they live in a dark, air-conditioned man-cave.
Hope that helps. Let us know what you decide.