You need not spend a fortune on clothing, I am most comfortable in poly/cotton trousers, the BDU type in green on my legs.
I prefer cheep as chips cotton T-shirts on my body. People argue that you can sweat up in them and then freeze as they don't wick moisture away but if you're going to sweat, just take your jacket off. They're also light and if one gets burned or covered in bunny guts it's not throwing Ģ30 in to the fire
Boots are important as you typically spend all day in them for long periods, it's worth buying these from a shop so you can really try them on, sizing is diferent with each maker so it's better to be sure... there are a million arguments out there, gore-tex or not, leather or textile or whatever. Personally my feet sweat a lot and I find gore-tex too warm, I want boots that will be a bit airy, sure they let in water but they also let it out easily too... it's all down to preference.
Socks are important, I buy the best I can afford and try to avoid ones with seams etc, socks are mostly overlooked as they don't come under the glamour status of jackets etc...
For a jacket, I wouldn't be without my M65, it's really warm in the winter and the lining comes out for the summer, it's bomb proof but not waterproof.
For rain, I use a US issue poncho because that's just my preference, you could really go with any breathable waterproof but I like the poncho because I never get clammy in it, you get reasonable air flow.
Buy a few hats, one with a brim for the summer, one warm for the winter and you're ok, no real hard and fast rules that i'm aware of, just that it should fit. Some people like leather in the summer, I don't because I don't want to lose a Ģ65 hat.
I would advise a good leather belt to hang all your kit off (knife, pouch(es) etc...) and a nice comfy fabric one for holding your trousers up with as this allows you to remove your kit easily for sitting and being comfy.
I have a top down jacket that hardly gets used and a few good high end waterproof jackets that never get used... i'm sure there are some other really good garments out there but they can be a bit too high-tech and fancy, I like to keep it simple and cheap. Don't be afraid of bright colours, if you're wearing all green, try a bright orange T shirt on, you never know when you might want to be seen
My main advice is to listen to other people but go your own way, many people, including myself, like to do things their own way and it may work... but maybe not for you.
I knew it was raining cats and dogs because I stepped in a poodle.