I use a layering system which works very well for me in all seasons and all weather on general hill walks, with an occasional mountain thrown in:
Baselayer (cheapo anything from Aldi, sports direct, M&S etc.) Don’t think I’ve paid over £7 for any of them. It’s the material it’s made from that’s the key, not the brand name, I’ve found.
Fleecy mid-layer (cheapo anything from Aldi, M&S, Debenhams, M&Co, etc.) Don’t think I’ve paid over £20 for any of them.
Insulated (padded) jacket (not waterproof, at best shower resistant, anything will do). This also serves as a windproof when not raining but chilly. I paid £25 for my current one.
Waterproof outer shell (this also acts as a windproof over a fleece when the insulated jacket would be overkill). I paid about £150 (I think – had vouchers and blue light discount and haggled). It has proved its worth as a breathable, windproof and waterproof. Money well spent in my opinion. It’s currently
on sale too.
Previous waterproof shells I’ve had have been the laminated boil in the bag kind and, yeah, they keep the rain out but also keep the sweat in, so you feel cold and clammy from the sweat.
I wear thermal leggings (currently M&S ones) under ordinary walking trousers, with waterproof over-trousers when the wind-chill is bitter.