I want to grow it INDOORS near a 10-foot tall window that gets plenty of light but never direct sunlight. I want to keep the tree between 4- and 6-feet tall.

My local nursery can special-order the tree from a wholesale grower near Santa Barbara, California -- about 160 miles from my home in Riverside, CA. (The tree is NOT imported from New Zealand).

Should I order the tree in a one-, three- or five- gallon can?

How can I be sure that the grower will deliver a decent-looking plant?

I don't know if the grower grew the tree in plain garden soil (i.e., "dirt"). Since garden soil is not recommended for container plants (especially, indoor plants), should I wash the soil from the roots and re-plant the tree in a good-quality potting mix? If "Yes", how can I do this without killing the tree?

Since you're planning on keeping the tree in your house, you should probably go for the 1 or 3 gallon..just to see how it does. You don't want to spend a small fortune only to have it die..Is this tree hardy to your area? The reason I ask is in case it doesn't do well inside, is there a way you can set it outdoors?
As for garden soil, trees are use to certain soils. Perhaps you can replicate sterilized soil that the tree is growing in..or what it requires. Sorry, I know nothing about this particular tree, so don't know what type of soil it prefers.
Also, many times, plants are shipped in light soil for cheaper shipping charges, so you may need to ask someone at the nursery which type of soil it prefers. Then by using sterilized soil, duplicate it.
One problem keeping the tree in outdoor soil is, bugs..you might find all types of outside bugs in the soil, and I'm sure you don't want them wandering all over your house..lol..I wouldn't blame you.
Hopefully, someone at the nursery will have info on the tree..you can also try typing the name in your search engine to see if there's any information available.
I've washed soil off roots many times..just be careful when removing tree from its pot..You don't want to break roots. Then, using a hose, spray away. Discard old soil. BTW, what type of pot are you going to repot in? I wouldn't use any decorative types, especially one w/o drainage. Even though plants are shipped in ugly, plastic pots, plants usually do best in them. If you have your heart set on a nice pot, you can always set plastic pot inside decorative one..but set something on bottom like a small brick or pieces of styrofoam sheets..That's what I use. This way when you water, roots won't sit directly in water. Good luck, Toni

Be careful, I've purchased a few "container" plants that were just dug from the field and plopped into a container.

A quick google search suggests to me that it is not too picky about soils. I would expect that you would be OK with any container medium that retains good structure. The container medium from the nursery may or may not be suitable for long-term planting. Sometimes they use peat-based media since they are pushing the growth and potting plants up so quickly. Tapla's container media threads on the Container forum have all the information you might need on building a long lasting container media.

When repotting, you shouldn't have to wash all the soil off until you have a "bare-root" specimen. In most cases, just washing much of the soil off from around the edges will be sufficient.