Probably best to go with heavyweight wool - if you want to keep the cost down have a look at ex-rental kilts. I purchased 2 of these for when I work as a tour guide (one from McCalls and one from Slanj) - similar reasoning to yours, I didn't want to wreck my good kilts but wanted something that was durable, which I would be happy wearing but equally not mind it being thrown in a case or covered in mud on the trail - and didn't cost a £400 - I think I paid about £150 to £200 for the ex-hire ones.