I'm probably wrong again, because of my understanding of xc skiing and this what most of people here understand as xc skiing :) So I will go from my way of xc skiing (on groomed tracks, similar as xc ski racing is). It depends on your fitness level and xc ski technique and abilities, but as soon as you get to real skiing and not just walking on skis, you will most likely need more then one. With alpine few degrees up or down are not that critical, but with xc ski, and your heart rate high up, there's big difference how you dress at 0c, at +5c or at -10c. And something what might be great at -10c, will be super hot at -2c, and for 0c and warmer, I'm normally just in xc racing suit with some sleeveless jacket on top and t-shirt (Swix of Craft underwear under), while when it gets below zero (in celsius) I'm switching between 2 "warmup" jackets (one thin and one thicker) for top with race suit and long sleeve underwear under it.
As far as what to buy, try to find some for normal people less known companies, which specialize in xc ski gear, like Swix, Craft, One way, Odlo... They have products specially made for xc skiing, and it works better then general stuff.