You don't say if you're climbing (and hence maybe hanging around on belays for longish periods) or walking (and hence staying on the move pretty much most of the time). Glyders' advice is pretty spot on for walking I reckon.
My experience is climbing - Cairngorms mainly, at any time of year when it was in nick, and Wales when the conditions and weather were mint and warranted a quick overnight drive and a quick walk in / route and return the next day. The latter I'd just wear a Buffalo type top over a baselayer, carry my shell and do the whole day like that and open zips etc. to vent. Legwear wise I can't remember as I had a few different things but generally it was powerstretch tights under a 3 layer shell bib (keeps you dry wading through snow, kneeling on turf etc. and is durable) or Buffalo type bibs for cold days.
Scotland was always a much bigger day, and I'd do it whenever - so even a sunny day in January could become a -13 night by the afternoon, although the kit wasn't too much different. Never rode the funicular and walked up the ski tows up to the plateau, 4am starts to get on the route at first light, so it was a hot and sweaty slog in - so I'd take a spare baselayer, take off the Buffalo and wet one at the start of the route / before putting on crampons, and change into the dry one (unlike Glyders, but you do it quick!), put the Buffalo and shell on, and crack on for some long, cold spindrifty belays interspersed with some pitches. Generally fine in the clothing, cold in the feet - I guess that's from wearing La Sportiva Nepal Tops, bought then before the insulated extreme version was available and never had plastic boots.
A couple of things I was taught were really important - one is gloves. I worked in a shop so had top notch waterproof leather palmed gloves for climbing in (that I could afford at trade price), but had spare liners and Buffalo mitts in the jacket - spare liners just in case a pair gets shredded or ends up wet, and the mitts to put on over liners whenever there's no need to use fingers.
I'm also pretty sure I never stayed hydrated or ate enough, and occasionally let my fag papers get damp - and learned that butane gas lighters are useless in the cold. I don't smoke now but on one occasion after a 14 hour epic in the Loch Avon basin managing to roll a fag in a whiteout with a wet paper and get it lit felt like the difference between walking out or laying down in the snow.
Oh (not assuming you can't already but it's always worth repeating), and make damned sure you can read a map, and walk on a bearing whatever the conditions. I never owned or had to use one but we always had one in the 'sack - a bothy bag big enough for your party is probably essential. As is a headlamp that's going to last the duration, and a basic first aid kit - plasters, lyon survival bag (although they say they're useless in the wind) painkillers, Victorinox Classic penknife, triangle bandage, wound dressings and a roll of Strappal tape , in a waterproof bag
If you're camping, keep all your stuff in a drybag and use a bivi bag over your pit to keep the condensation off, and consider some kind of 'hut bootie' so you can go outside and not put wet /cold boots on. A layer of aquasure or seamgrip on the soles is a good idea.
Also worth getting into the pub at the end of a day if you can to dry off in front of the fire.
Edit: George's comment about a belay jacket is a good one - I couldn't afford one at the time so never carried one, but it'd have been ace. I reckon if I was going to, I'd look at a Patagonia DAS Parka. Bloody expensive though, but Scottish winter is a serious business. I guess guys did it (and Everest for that matter) in tweeds and woolens, but you don't have to...
I'd say (actually answering your post in the first place) with the kit you have, stay sensible, don't go too far, make sure you know exactly how to get back home / back out quick, and don't go too high / into the middle of a plateau where it could get very nasty
