Basic kit that will make you look Highborn is a white or black shirt, black trousers, a black or white tabard (or group colours once joined) and a hood.
As far as cheap options go check charity shops. Plain trousers are easy to find, Linen good for summer event. A plain shirt in a medieval sort of style may take a little more looking but they still turn pretty regularly, especially if you check the rails for the other gender also.
The tabard you can make if you have any sewing ability at all (can thread a needle), or a friend who can. it’s basically a rectangle with a head hole in the middle. Drill cotton is pretty easy to get from most craft places, and you will only need a metre of fabric to do it.
For the hood you can use a large plain scarf, look for the wide, thin wool ones, probably in black or white, drape that round you neck and shoulders and have a fold to pull up to cover your head. Again charity shops have them all the time. Or also not that complicated to make.
That will get you in character and recogniseable as Highborn.
There are many traders who will do new larp styled shirts on site for less than £20, and a fair choice for tabards/surcoats also.
Spend your money on your boots. they need to be ankle supporting, waterproof and comfortable for several miles of walking.
You will probably also benefit from a leather looking belt (charity shop), with a belt pouch if you can (£10 from traders), and possibly a shoulder bag (charity shop).
Cloaks are fine for unconquered. Probably not on the battlefield as you’ll get tangled up, but around Anvil fine and definitely in the evening. Black or any other dark shade will do (navy, dark grey, brown or green). Cheapest option is a blanket, and a big safety pin hidden in the folds. Army surplus usually has good options.
Armour is more complicated. 2nd hand is cheaper, several facebook groups for that, but for the first few events you’re probably better asking to borrow. Several of us have more kit than we could possibly wear and with a bit of notice will be happy to put a set in with the general packing.