I took a similar approach when putting my formal outfit together. This is the result:

Kilt, 8 yard, hand sewn (by our own Barb T)
Argyle (NOT PC!!) jacket and waistcoat in black (more options)
Sporran appropriate for evening wear
Ghillie brogues
Tartan hose (still jonesing!)
Belt + buckle

This list is in order according to my prioritization method.
The kilt is first because, well, it's the kilt.

Then is the Argyle jacket and waistcoat because I found a good deal and they are another large chunk of the outfit. If you want to go less formal, just get the jacket, skip the waistcoat.

Then the sporran. This is the next largest "deal" to get over. Be selective, there are enough people making sporrans that you can get pretty much exactly what you want. Do not, I repeat, do not get a "semi-formal" sporran, if you want to go out to dinner in the evening, get an evening sporran.

I listed ghillies here. I already had a nice pair (piper's with extra cushioning due to my spinal issues). They are ubiquitous, and are thus easy to find. Shop around a bit, you'll find the exact ones you want with a little bit of searching.

Tartan hose. You might want to step down a couple of notches, go with nice (read: hand-knit) colored hose, or maybe even some Lewis hose. Those will work until you start to get fairly formal.

Belt + buckle. These are easy to order, so no hurry. You can get them on sale if you shop around for a bit. The same buckle will be sold by several vendors for several different prices. I say get a belt and buckle because the jacket without the waistcoat paired with a belt and buckle is a very nice, a bit less than formal, look.

That's pretty much what I did. Pay special attention to the sporran and hose, those two items, more than anything else, can make or break an outfit.

Kilt on brother!