Nick
Great idea for a thread, I'd have to say the essentials really depend on what you want to look like when kilted. For me anyway, the following would be my list:

Essentials
1. A Kilt, in my opinion the best kilt you can afford is the way to go.
2. A plain leather Sporran, a Freelander would be a good choice as it holds quite a bit and looks great doing it.
3. A pair of Hose in a colour that compliments the kilt I'd say for a start W. Brewin hose are a great buy.
4. Flashes or Garter Ties, I prefer garter ties as they are more comfortable, available at the Scottish Tartan Museum.
5. A Belt either one of those Dress Belts or a Casual Kilt Belt, it really is your choice again it depends where and why you will be wearing your kilt.

Secondary Stuff
1. A Sgian Dubh, I've found some great buys on ebay for Sgian Dubhs
2. An Argyll Jacket, I'd say for a start a Black Barathea with silver buttons as it will be the most versatile, I'd say go for the waistcoat here so you can dress it up.
3. A Bonnet of some sort either a Balmoral or Glengarry, in my opinion a Balmoral would be best.
4. An assortment of solid colour ties, and a Black bow tie, to dress up your Argyll Jacket from number 2.