SSP Rarity/Values

[SSPGARY]

I hope this is the right section for this. I'm fairly new to SSPs and was wondering what makes one SSP more rare than another, if any. I have been into SVOs for years and know that certain years, colors, and options make all the difference from an ordinary car to a rare car. I'm looking at buying another car by this spring and was wondering if a marked or unmarked is worth more money, high mileage or low mileage, Postal or Law Enforcement, never in service or..... you get the picture. I now have a 42000 mile unmarked Santa Clara unit that I bought because of the fairly low mileage and excellent condition ( a good start to my collection ). Seeing as it costs so much to restore these cars to original in service condition, I thought it would be nice to restore a rare SSP. Is one color more rare than another or one state than another? Any input appreciated.

Gary