If by historical, you mean the Great Kilt or Belted Plaid, I would avoid going that route. Those are best left to historical reenactors. There are modern tailored kilts that are inspired by historical kilts, such as the 4yd box pleated kilt. These are gaining popularity as an alternative to the traditional 8yd knife pleated kilt. You sound as if you are beyond high school age since you mentioned missing prom, so I'll assume you won't get much taller.
Look towards your family to see what you future body might look like. When you choose a kilt, do your best to work directly with the kiltmaker instead of a middle-man. See if they can build the kilt to allow for a few inches of future growth. Don't rely on the straps to last you a lifetime of adjustment.

Next I would look for a nice plain sporran. Black or brown will be fine, but black will let you slide into evening functions, whereas brown is typically worn only during the day. Plan to spend a couple hundred here. A quality sporran can last you a lifetime.

After this you might only have around $200 left if you started out with $1000. So go ahead and by a few pair of hose. Lovat colors are typically worn during the day. Bottle green is a good color that works both during the day and evening. You can get by with one pair of flashes. Red is probably the most versatile.

Whatever shirts and shoes you have now will be fine. You don't need a sgian dubh. I wouldn't worry too much about jackets and waistcoats until you are able to purchase one on your own. I would avoid the typical kilt belt with the large rectangular buckle. Any belt two inches or wider would work just fine. Just make sure it is the same color as the sporran. Or you could just forgo the belt altogether. It really not needed.

Keep us posted on your progress and keep asking questions. Encourage your brother to join us as well.