Welcome to the forum!

I would echo what others have said here: the hose and sporran are the most critical. I have a bunch of issues with allergies and can't regularly wear wool anymore. If you can, go for it - wool hose will likely wear less than synthetics. But if you want to avoid wool, check out this thread I put together on the subject: http://www.xmarksthescot.com/forum/f...-review-96885/

Sporran - OCR's thread is great for this. Get one with enough internal space and you won't miss pockets. But if you don't want to go used, there are lots of good options for new sporrans for reasonable prices.

Now, the more optional things. Belt and buckle. As has been said, on a properly fitting kilt, the belt is essentially decorative. I like to wear one to add a visual break between the kilt and shirt, but to each his own. If it's a functional item, you're doing it wrong.

Now, I'm going to go a little against conventional wisdom here and say that I do find a kilt pin to be useful, particularly with casual kilts. It does tend to keep the front apron from flapping around in the wind, and I do get comments on my kilt pins as a decorative item. But I'd say that's likely only true of a casual kilt. I don't wear a kilt pin with my semi-trad because it just doesn't need it - there's more fabric on the right edge of the front apron, and it adds enough weight and structure to hold things in place without a kilt pin. A kilt pin is pretty much always considered optional. But I personally find them useful for my casuals.

Flashes: again, I find these to be quite useful. Indeed nearly essential if you wear cotton hose instead of wool. Cotton hose just don't tend to stay up well without them. And I find that my calves do stay warmer in the winter with the flashes essentially sealing the tops of the hose. But...to each his own.