What should we be looking for when evaluating the suit? What effects would 10 years have on a vulc rubber suit? Anything we should be wary of? Do you think a rubber suit of that age could still be functional? My guess is the seals probably need replacing, but how does the rubber hold up?
You're asking all the right questions.
The first place I like to start is to look at the bottoms of the boots. That'll tell you how many overall "miles" are on the critter (but not necessarily its condition).
A well maintained older suit is preferable to a newer suit that hasn't been taken care of.
Next look at the zipper. Is it clean, well fed & cared for?
Look for the inevitable loose threads along the edge of the zipper. Those can be cleaned up, a snip with manicure scissors & singe with a lighter takes care of them, but too much fraying starting to approach the teeth is not a good sign.
All the teeth need to be straight & tight. Some models have the HD (Heavy Duty) zipper installed, they have 6 teeth per inch. Regular Viking zips have 8. The HD teeth are actually riveted on, a close look at the tooth will show a tiny round spot, which is the rivet. "Regular" teeth are crimped on.
Slowly run the zipper closed & open, checking for smooth operation. You should expect some resistance, it's normal. Uneven operation can reveal dirty or misaligned teeth.
A well maintained zipper on a Viking should easily outlast the suit.
Look at the suit's rubber; checking for "gummy" or cracked spots. Look for discolored spots, examine those more closely. Scratch with your fingernail.
A suit used in straight oil can have its life very quickly shortened; the rubber will start to soften & eventually come off its backing.
Check for stains inside the suit, that can indicate persistent leaking. Crotch areas are a place to be more thorough with your eyeballing. Bad spots on the rubber are a deal breaker, there's little that can be done to fix that.
Next come the cuffs/seals. They should be stretchy, snapping back to original dimensions when stretched & released. They'll show the most common deterioration as a gumminess.
Check where the cuffs/seals attach to the suit material, cracking shows up there first, usually. Well cared for cuffs can easily last 5 years or more.
A well maintained & properly stored Viking will easily last 10 years; I have some going on 20.
Check the valves for smooth operation. Some stiffness/sticking will be normal for a suit that's been in storage for a long time & a careful shot of silicone should loosen the critters up.
I'm probably forgetting something here, feel free to chime in.