Twonius, I would avoid buying used if at all possible. Build quality has gotten much better and there are many design refinements. For example, 2013s have loud compressor pumps, lack the hill hold feature, have a back-assward arrangement of cruise control stalk and turn signal, feature inferior interior roof materials, and are in general more rickety. It's also missing automatic braking and AP hardware. In my opinion the current Model S, even before the refresh, is a very different car from the 2013 version.
I just don't think the value is there for going used. My feeling is that used cars tend to have options that don't really matter too much, while even baseline new cars will have important safety and design upgrades that will make a difference - at least to me, anyway.
So I suggest buying the Model 3, or the baseline RWD S with minimal upgrades. Consumer Reports just told us that tires, not AWD, are what really matter in winter, and who absolutely needs that rear power lift gate, anyway?
In my mind, Teslas are like iPhones - if you're going to pay up for a better product, get the latest and greatest, even if it's the cheap model. And they both will change your life.