Very cool deal. Just a word of advice I wished somebody had given me earlier though -
What you pay for now in cash, you make up for in time. Material like this is GREAT for learning and experience, but when you want to really start practicing bonsai, I find that it's usually worth a bit extra to find material with good trunk thickness and good nebari. Those are the two most time consuming parts of bonsai, but if you have a tree with both of those, you can really work on styling. From what I've seen, it takes a lot of practice and experience with real bonsai to be able to take box store plants and make them into works like Vin does.
A helpful list of bonsai ideals can be found here:
https://www.evergreengardenworks.com/rules.htm . Think of them more like suggestions for traits that make a bonsai look more convincing, rather than straight out rules though. One of the first things that list mentions is that height should be no more than 6 times the width of the trunk at the root base. For your tree, that would be around the green line.
Take the following with a grain of salt, as I'm still relatively new to this as well:
If this were my tree and I wanted a real bonsai out of it, I'd plant it in the ground and wait a few years. If I wanted to use this for practice (and at that price, why not?), I'd cut at the red line. This would make either one of the blue lines your new trunk line. I'd wire it and see where things went from there. I've never worked with a bottle brush so I'm not sure about the growth habit, but it may back bud lower. If it does, it might chop it again lower after the next growth season.
Also remember that every plant has an optimal potting season. The reason why slip potting was brought up is because most bonsai are best repotted (meaning bare root and root trimmed) in late winter or early spring. Luckily, bottle brush looks like it may be a sub-tropical type, which can also do well if repotted in summer. Be sure to double check for any new bonsai though!
Helpful links:
http://www.dlnursery.com/d&l_nursery_055.htm
http://www.ausbonsai.com.au/wiki/index.php?title=Callistemon
It also helps to look at other members' progressions:
http://www.bonsainut.com/index.php?threads/bottle-brush.19696/
http://www.bonsainut.com/index.php?threads/bottlebrush-bonsai.12812/