Tread depth is only part of the equation.......If they're "older" like you say - how old are they? The rubber compounds harden significantly with age, especially if they've been run too long into higher weather temperatures. Hard tire = crappy winter tire, no matter what the depth of tread. If you're serious about buying used - check the production date - it's stamped into the sidewall. I wouldn't touch anything more than two years old.
As to studded - like previously mentioned IF you're driving a lot on icy highways, go for it. I've aways found with studs they're definitely the best for ice but for every day running primarily in the city or on clean/dry roads they're a detriment. When the stud touches the pavement it actually pushes the rubber away from the road. Makes the tires feeling quite "slippery" on anything but ice. (This of course is only my opinion).
I just recently changed cars(and had to change tire size for winter). Bought the exact same make and model of tire and it's INCREDIBLE how much harder the (only) 3 year old tires were.