I'm going to contradict what wildrover said... #1 is a personal decision. There are no kilt police who can tell you what to wear and not to wear. Wear whatever you want and whatever you are comfortable with.

Having said that, I would point out that many of the ex-military kilts you may see on eBay would be fine for just about anyone to purchase and wear, especially if it's in a Black Watch tartan (sometimes called a government tartan). Black Watch is universal, meaning that you don't need to be in any particular clan, nor military, nor ex-military to wear it.

Put it this way. I have a Black Watch tartan kilt, and I am neither Scottish nor military and no one has ever said anything negative about it or me in regards to "entitlement" to wear it. In fact, no one has ever even asked me if I belonged to a Black Watch regiment. I only ever get asked if I play bagpipes but that's about it.