I am going on a Ski trip with two days in the snow. Given that I am unlikely to go skiing again for a long time, I would like to minimise the amount of gear that I purchase. The minimum temperature appears to be about -2 or -3 degrees Celcius. How important is it to have thermal underwear? Is it something vital, or can I wear something else instead?
No, it's not vital to have thermal underwear. Layering is your friend.
During the winter months, I've been hitting the slopes regularly for the past 8 years or so. Though I do own thermal underwear, I've rarely had to use them. What I normally do is simply layer my clothes and then add/remove as needed to be comfortable. The downside to this approach is that, when it's really cold, wearing all of the clothes could get pretty bulky.
In winter your worse enemy is sweating. It is really hard to deal with it once you are sweating.
An appropriate thermal underwear will help evacuating sweating instead of retaining it. This been said, an appropriate winter jacket is also essential to keep you warm and also evacuate the sweating from the the internal layers.
It is not necessary to pay a lot to have a good thermal underwear.
You did not say whether you were going downhill skiing or cross-country. The difference is significant:
When downhill skiing you are being carried uphill each time between runs and so are not generating much of your own heat and so are not going to sweat much. You are also never very far from warm shelter where, if you feel cold, you can usually (a) consume hot food and drink and (b) blow hot air into your (temporarily empty) gloves and boots. Thus, so long as you have enough clothing to keep you warm, it needn't be "thermal". In fact, it is even safe to wear (the dreaded) cotton next to your skin. I do.
When cross-country skiing -- or snow-shoeing or winter hiking -- you must propel yourself up every hill and doing so can generate much heat and cause sweating or giving off moist air. Because you are going to be relatively far from shelter, it is critical that you have "thermal" underwear and breathable outer shell that helps keep you warm and dry. Synthetics or merino wool will do.
If you don't have proper base layers, you may be regretful during that 15-20 slow lift ride while you're sitting in your own sweat. You do not need to buy base layers made by a ski/snowboard company to get the benefit. Also, it's not warmth that you get from this layer but a place to wick sweat away. I use my snow base layers as the coolest t-shirts I own in the summer.
Good luck and have fun!
Good underwear is very important in winter conditions. When you're active, you're sweating. The moisture greatly reduces the termal isolation, which increases the chances of hipotermia.
Cotton is very bad for underwear, because it absorbs moisture. When it is wet, it acts like cool compress, draining the heat from your body.
Some people find cotton fine for them, but those are the people who sweat very little (I've noticed they drink a few, they have the first bottle of water half-full when I'm finishing the second etc.). But for most people, like for me, cotton is very bad as underwear in both summer and winter.
You don't need the underwear that have the label 'thermal'. You just need synthetic underwear, because synthetic fibers absorb little moisture and they get dry quickly. Wool is also said to be great, but I've never tried myself. It is told to scratch a lot ;)