Trousers won't matter so much generally, you want to get the right top half obviously but that's going to be dictated by your LBE more than anything. I've got some older style plate carriers with no 3D spacer mesh that would make anyone sweat like mad in even mild temperatures, but in the cold it keeps all the warmth in to the torso so you can quite easily wear it with a UBACS (presuming you do actually run around playing airsoft that is, there are people out there I've seen who find that too much of an effort).
If you're running a chest rig, a thin tac vest or a PC that you find actually stays quite cool in the summer, then look in to some more layers; preferably something in nice modern materials like a HH or UnderArmour, something that won't soak up sweat like a sponge as standard civvie cotton clothes will (i.e. a big baggy sweatshirt/hoodie).
Gloves are pretty important to consider. I've got weird, rubbish circulation to my hands, they'll go numb and start hurting like hell when other people might be fine, so I wear my issue leather combat gloves; totally waterproof, very warm and you can still manipulate a weapon (if more slowly than before). Remember to switch out your cap/boonie for a wool or fleece beanie if you are feeling the nip, that always looks afghan-mountains tactical, lots of cool options out there. Last but by no means least, good quality, cared for, waterproof boots with decent socks; preferably ones with actual padding inside for insulation, unlike the old standard issue boot which are purely leather.
