Warm Clothing for Cold Sites
There is little point in buying special clothes to go excavating in. Anything will do so long as it is warm and you don't mind getting it dirty but, if you are outside all day, and especially if you are crouched excavating in a cramped hole, then the cold can really tell on you. This is particularly so in windy conditions. In the wet, even thick cotton provides very little resistance to cold. Wet wool smells and is itchy. What you want is one of the modern synthetic fleeces. They are light and compact, don't smell when they get wet, and the better ones keep out the wind a bit as well. There are lots around now, and they don't need to cost the earth, but I would go for a reasonable quality one because it will be warmer and last better. I think a good fleece jacket is one of the basic requirements for fieldwork. Get one with a zip down the front so you can control the temperature. They are really useful for travelling anyway - air conditioned aircraft and airports can be chilly - and don't forget to take one for the cold nights in places like North Africa. For cold climates in winter, you'll need a warm hat too because you lose a lot of heat from the top of your head. Woollen hats itch and I would suggest a synthetic fleece one. You'll also want gloves if it's cold and, again, fleece ones are good. They get hard wear on site, so I wouldn't get the most expensive. Some have sticky plastic patches on the palms, so you can grip things, but it can still be difficult to grip a trowel or operate a theodolite properly. They make fingerless gloves which work quite well for this situation. Synthetic fleece clothes still keep you warm when they are wet, but make sure you have at least one change of dry clothes especially if you are camping. Take a large plastic bag to keep them dry in - even new tents can leak or become engulfed in a flood.
Trousers take a beating on site, because you're kneeling in the dirt the whole time, so it's not surprising people go for market stall/surplus style. Favourite dig wear right across Europe are those grey-green army style heavy cotton trousers with lots of pockets. As I say above, cotton is not the warmest thing in the wet and it takes a long time to dry. For really horrible cold wet conditions, you might like to try "technical" walking/trek style trousers made of synthetic fabric (they still have all the pockets if that worries you).