The cheaper setups can give you a good taste, and they have reasonable resale value.
Stax bargain hunting outside of the confines of the head-fi forsale fora is tricky business for two reasons.
1: If an electrostatic hasn't been used in years, it probably needs a few hours of power before it will wake up. Also, people who don't understand the transformer boxes ("energizers") that hook up to a speaker amp often hook them up wrong, and get no sound at all. So sellers will often list a potentially working item as broken.
2: Sometimes they really are broken, even when the seller says they work perfectly.
Sometimes, you get lucky. My SR-X III + SRD-7 cost me something like $98 shipped, and works perfectly. Needed new pads that cost me $25 from a different ebay seller. Apparently a lot of SR-X 3's have one or both driver on the way out, but mine doesn't.
Sometimes they're easy to fix. Sometimes the channel imbalance is just a wiggled connector away from being fixed. And sometimes it's a gonner.
Sometimes, if the seller doesn't know if they work, you can help them hook them up for a proper test. But they may have to leave the energizer turned on with the headphones plugged in overnight before they work properly.
My only other advice is not to pay too much for stax electrets. I don't think any of them are worth more than $100, unless they're in literal NOS condition. The SR-30 (which is SR-60 in europe) and SR-80 are also fairly fragile, unlike the rest of the Stax line.