Bear in mind that neither Wintermark or Navarr are Celtic, there isn’t a big thing on tartans…
That said, an intricate pattern in subdued colours certainly fits Wintermark. Material…? What’s it made from? Because as long as it isn’t too obviously synthetic you’ll be fine.
Re costume on a budget, charity shops can be great sources of stuff. Trousers, tops, skirts, etc, something that you could tweak or change with a sash or a simple design.
As for boots… as Thiromar said, your comfort is more important than the appropriate nature of footwear. Floruescent pink wellies are probably a no, but any sort of sturdy boots are great.
Walking/hiking boots, wellie boots (which you can decorate), even the more muted and sturdy sort of trainers (Sketchers, for instance) are acceptable.
However, if you’re planning to come along in April, and you think you might like to do a few LARP events, I reccomend paying for some good boots of some sort, if you don’t have any. What would you wear if on a walking holiday…?
Oh yes, and as Hob said, keep an eye out online and you might find quite a lot of stuff being sold pretty cheaply.