Since you pretty much have it wrapped up from the waist down I would say that a jacket or two would be in order. A regular argyle, braemer or crail is a good all-around choice as it can be dressed up for all but white tie affairs or worn more casually. If you have only one jacket go with black. If you can do two make the second a tweed or something else well suited to casual and daytime situations.

Of course there is always the option of converting a jacket. I have done three and am very pleased with them. One is very similar to a black argyle, and good enough to be worn next to someone wearing a commercially made one. Another is a gray casual sport coat type thing and the last is a short jacket similar to an "Ike." I got them all at thrift shops and have very little green invested.

There are probably a couple dozen good threads on converting a jacket on this site.

Shirts can be anything from an off the rack Wally World dress shirt to a custom tailored piece costing hundreds. Ties, if desired need not be tartan. Plain colors look great and can be used to compliment or set off colors in your tartan. Fancy or flashy ties don't really work well. Tartan ties are fine if you like them, but it must be worn with a kilt that matches or with a regular suit...mixing tartans just doesn't work for most.

Other pieces of your kit can be picked up as you go, such as unique kilt pins, fancy sporrans, and the like.

And the polo works well in casual settings. That's actually why I have so many. T-shirts are even accepted in many situations.

Try to find pieces that can be mixed for different looks and levels of dress. Two jackets, four shirts, a few pairs of hose, two pairs of shoes, and a couple of sporrans can set you up for almost anything.

Panache put up a really good post a while back showing 10 levels of dress ranging from very casual(hiking, backyard barbecue, etc.) to white tie. You can get some good ideas there, as well as from several other threads featuring members' pics including casual wear and formal.

Bottom line is let the kilt stand on it's own, and there is no reason to break the bank building a wardrobe. Just remember that you don't have to look like the rental shop pictures, or something from a catalog. You're off to a good start from the sound of it.

Hope that helped a little.