Unfortunately theres a lot of bullshit and fashion taxes in the outdoor clothing world and no shortage of businesses prepared to relieve you of large sums of money for things that don’t work.
A few things to bear in mind:
1) Waterproofs when doing heavy exercise will always become sweaty inside so vents are a must, no matter how technologically advanced the manufacturer claims the fabric is!
2) The nature of MTB sweat, mud, grit, abrasion, constant washing, crashes etc mean outer layers get trashed and waterproofing gets worn away over time, sometimes very quickly.
3) The reason most gear under performs is because of the compromises required to make practical clothing i.e. weight and bulk for biking, the constant abrasion damaging the fabrics and wetting through from both sides.
4) The big established names often charge way over the odds for their most basic low end ranges, which are no better than other cheapo brands, be careful!
5) Apart from neoprene once a fabric is saturated (including unicorn wool Merino) it’s insulating properties no longer make any difference, think how fast heat transfers through a wet tea towel when you pick up a hot dish from an oven!
Having a well insulated windproof and ventilated core/trunk, dryish crotch, warm (not necessarily dry) hands and feet go a long way to making you feel more comfortable over long periods. You will never be completely dry or 100% comfortable and some of your gear (particularly outer layers) will have to be regularly replaced or re-waterproofed due to washing and damage.
IME A water and windproof ventilated jacket is most important because non of the layers underneath will work if piss wet through! Don’t spend an absolute fortune if you fall off a lot, you will be sick when your 300 quid coat is ripped first ride! having said that you won’t get away with the cheapest non-breathing fabrics when weather conditions are at there worst! Synthetic base layers tend to be more effective and cheaper, although some are not as comfortable against the skin. You can get away with the cheapest polyester ski base layers for a few quid (I do) if it doesn’t irritate your skin when damp! I vary mid layers for temperature control, buying synthetic jerseys etc that can be layered and worn on their own in summer, so in simple terms anything thats not cotton from any sport will do.