For wintermark Kallavesi your minimum costume is going to be some variety of plain dark coloured trousers, summer weight linen would be nice, and a tunic top, slightly weathered, so holes, tears are OK, and a bunch of beads/feathers/whatnot as decorations. Check charity shops, look on the women’s side too and if you have enough time you should be OK, otherwise Primark, George at ASDA, Matalan and similar may have options, again check the women’s side.
For cooler events a furry coat of some kind would be good, also charity shop find, or if necessary a blanket (furry) and a big pin, again charity shops if you have time you can get some really good bargains.
Spend your money on footwear, you need decent boots that will support your ankle and survive mud if necessary.
Ask around your mates for the camping gear - minimum 2 person tent, sleeping bag, roll mat, which should be free or a couple of beers.
Budget for food, you can probably live off snacks if necessary - cheaper pre-purchased from a supermarket, even better if you can also borrow some form of camp cooking equipment to at least heat water for pasta pots etc.
You have listed a lineage as changeling which means you’re going to need to find a pair of ear tips and the glue to keep them on. They’re available on line is cheapest usually.
You have listed battlemage - I would suggest asking around your nation to borrow weapons, staff or rod will set you back £60 to £100 minimum. Worth also asking about armour to borrow too, but as mage armour can be made out of anything a bit of improvisation with some charity shop finds, perhaps belts and necklaces may work quite well.
Chirurgeon and apothecary roleplay can look better with props but there is no reason why your chirurgeon as a mystic can’t just wave feathers or make hand gestures over people to heal them (could look really cool), and poundland do travel pack bottles or sometimes craft bottles which could be redecorated as apothecary bits.
A decent belt, and if you can find it some sort of leather bag (also charity shop) would be useful on the field, along with a tankard for drinks and a water bottle for actual rehydration (if necessary 500ml sports bottle wrapped in a scrap of cloth will work). Plain notebook and a pencil also useful.