I will be walking up Snowdon in Wales, 1085m, this February and am looking for advice on what jacket and trousers to get. Weather supposedly is windy, may be snow-covered and probably wet. Should I go for waterproof jacket and trousers or general quick-dry clothing will be suitable?
The essential items are a jacket and boots. You need a decent waterproof and windproof jacket with a good hood. The jacket must be properly waterproof (eg GoreTex or similar) and not just water resistant. You should also have a decent pair of waterproof boots. If you're taking one of the popular routes and staying on paths/tracks then lightweight summer boots will be OK - so long as they are waterproof. If you are wearing new leather boots you might want to break them it a little first (less important for fabric/synthetic boots).
You don't need waterproof trousers, although they may make a wet day more comfortable/bearable. Lightweight trousers are best as they don't get too heavy and stiff when wet. For day climbs I wear Craghopper Kiwi trousers - they dry out from wet in 15 minutes.
If you encounter much snow/ice and are not experienced and equipped (ice axe + crampons) you should seriously consider turning back unless you have a trusted experienced leader who tells you its OK.
Other top tips;
Enjoy your walk!
If you are expecting snowy and wet weather then I would suggest following the concept of "layering", with a base layer of merino wool, and a hard shell jacket and pants (Gore Tex or similar) as the outer layer. If it is raining or snowing quick dry clothes have a hard time drying as the air is already very humid. Quick dry clothes are not typically very warm once they get wet, where as the wool will keep you warm even if it gets wet and is unable to dry (plus wool won't stink after wearing it for days). If its going to be snow covered, then be sure to get good boots and socks, and if the snow is deep consider getting gaiters to keep the snow from entering the tops of your boots.
Mountain Equipment Co-op has a great website for planning what type of clothing/gear you need depending on the activity. Outdoor Clothing Information
They also have trip checklists to ensure you haven't forgotten anything, they are based for hiking around Canada, but I'm sure they will still be helpful. The Skyline Checklist would be the most similar to your destination. Various Hiking Checklists
If I was going, I would bring something like what I brought for an overnight walk from Braemar to Aviemore in the Cairngorms at the start of February some years ago.
So apart from gloves, hat/scarf or balaclava - base layer top and bottom, hard shell jacket and pants, gaiters, a 100wt fleece and a 200wt fleece (giving 3 options for warmth) and for a day trip either hiking trousers or thin fleece trousers, the latter if you tend to get cold easily.
For a day trip I would add a thin water resistant nylon windshirt (100-200g) - one of those outside a fleece works well unless you have rain or wet snow. The much better breathability helps keeping you dry if it is windy but not wet, especially if your legs need to work a bit harder in snow.