Yes they are fast growing and bloom at a pretty early age. Some of the dwarf varieties are most likely slower growing but you don't really want a dwarf anyway if you want it to be a tree. I think it's a good choice. They bloom a long time.
Are you in Missouri or Arkansas? I was just wondering how far south you are.
Severe pruning or winter dieback like we get here in Southwest Missouri if we have a bad winter can cause them to sprout from the base more. Most of the standard crape myrtles we see around here look like big shrubs instead of trees. The hybrids that are more cold hardy might be easier to train into a tree form. I'm guessing that they might be less likely to send up a lot of sprouts from the base that you would have to prune out. That's just an asumption since I think they would be less likely to have winter dieback.
The chart below has lots of info and you can click on the cultivar names for a more detailed description and photos.

Here is a link that might be useful: Clemson - Crape Myrtle varieties

I had a seedling pop up (last year was the first I noticed it); it is over my head now. It has lavender flowers already. Ones that you buy take a while to settle in before they take off. Mine are all shrub shaped plants, but I also think it is a good choice. I have had no disease or bug problems with mine.

I have trees and shrubs and like them, and they all grow fast. I let my trees grow and I have also cut them back and they grow quickly back tall. I think you will be happy
with them. I have never had any bugs or insects
bother them. They come in several colors. I have red,
lavender, and white. Six in all, one is tall as my
house. I had some painting done and cut it back out of
workers way a year ago and it is already tall again.
Happy gardening. caroline