As I read everyone's responses, I'm thinking that a nice tweed (no waistecoat), a couple sweaters (one v-neck, for a tie... and one crew-neck for without), and a decent casual belt will sufficiently flesh out the wardrobe.

Most of my non-kilting wardrobe is either 5.11/Woolrich tactical type stuff (I'm a firearms instructor, among other things), and a handful of sport shirts on the casual side, two suits, one wool sport jacket, and some dress shirts on the dressier side, and that's it. For shoes, I own a pair of tactical boots for the range, a pair of deck shoes for the boats, dress shoes for the Coast Guard uniforms (I'm also a public education & boating safety instructor), and a pair of Oxfords for the office. My non-kilting wardrobe really isn't well suited (so to speak) for adaptation to a kilt.

I can do the extremes, with sneakers, a t-shirt, my Sportskilt and Stillwater cargo sporran on one end... and the Prince Charlie, with Fly Plaid, at the other extreme. But I think a sport-jacketish tweed, and a couple sweaters (with something more appropriate than a "Luxury Clan Crest Buckle", or my gun belt, at the casual end), would flesh out my options for the middle ground.

I think I have enough hats, flashes, and other related bits... my sgian is neutral enough to fill any role, and I think I can wait on any diced hose, etc.

So I'll go for the tweed, a couple good sweaters, and a good casual belt and buckle (thanks for that renleather link, Nighthawk!)

"Tanks" to everyone for the great input! It is very much appreciated!