Disregard the LS6 Build Sheet. The seller does by saying it can't be authenticated. If the seller disregards it, you can bet it's bogus.
An estimate is just that, an estimate. And yes, Insurance appraisals are typically higher than actual value but there is no standard percentage. You'll get a much better acurate real time figure right here.
The motor appears to be leaking and the seller states the booster and rear differential and transmission are also leaking.
25K sight unseen is going out on a limb at best.
This car is going to need work. All the leaks alone not to mention what's not mentioned.
25K should get you a decent driver with the same cosmetics or lack thereof.
But at 25K, it needs to be in decent mechanical condition and the body and Chassis need to be sound as it's the foundation which nice cars are built on. It dosent matter how much you fix or detail the car if the body and Chassis are sub-par.
The photo's provided would concern me without a physical inspection.
25K is tops only assuming that the body and Chassis are sound.