I would suggest looking for an owner, or evidence of one, that appears to have given a sh!t about and cared for his SSS. Mileage can vary, but responsible ownership is a must find. Otherwise, your hard earned $$$ will be forced to make up for the negligence of the previous owner. Instead of spending on improvements you'll be spending on unforeseen/neglected maintenance.
I was on the hunt for well over 1 year. Had a 2001 GMC Sierra C3 w/ 150K and some rust issues (bought it cheap (($12.5K w/ 75K mi in 2008)) in PA). GM's 1st attempt at a luxury sport p/u. Very similar and even more rare. However, I must admit, not as holistically well executed in design as the SSS. Found my SSS 2 months ago in MD. 1 owner 82.5K mi. Very well maintained, bone stock, and very very clean. Needed nothing. Asking $17.5K. I brought cash. $15K made it mine. All of my transferable upgrades (20" wheels/tires, tonneau cover, K&N, big stereo, ...) from my C3 were able to directly (and some eventually) be applied to my SSS. So all things considered, I am a happy man.
FMV on these disappearing vehicles is highly subjective. None of the value guides available will be able to accurately steer you right on price. They are literally worth what someone is willing to pay. Period. For example, the last SSS I looked at prior to my purchase was a 2004 Arrival Blue, garage kept, 1 owner, never seen snow, never seen rain, 42K mi cream puff. It was absolutely stunning. I wanted it. He wanted $28K. He wasn't flexible. He kept saying, "Go find another one." IMHO - Fundamentally what he said was right, but financially it didn't make any sense to me. Some of what he had to say rang true. But it wasn't $28,000 worth of my hard earned $$$. I walked away. No regrets. That deal wasn't for me. He did call a few weeks later to inform me that he sold it for $26.5K. Good for you, but whatever. I have no desire to be blindly/massively upside down in a vehicle.
I do believe good quality SSS's are currently going back up in value. However, I wouldn't want to be so deep into 1 that I'm waiting 5-10 years to break even with a whole lotta luck (read without unforeseen expenses) being involved. Don't get me wrong, I appreciate all 4 of my vehicles and treat them with loving care. I simply have no desire to hold onto and pour my $$$ into any 1 of them for more than they are worth.
So for what it's worth, my advice is to be patient, be persistent, and don't force yourself into settling for something rather than nothing. The perfect deal you are looking for is out there... you simply have to find it. Best of of luck.