I wouldn’t buy a bike before taking the BRC. Don’t worry too much about trying to get practice for the class the first time. All I had going for me was I could ride a bicycle, and I drove a 5-speed manual shift car all the time, so I was used to shifting gears. However, I had absolutely no motorcycle experience or knowledge other than my online research, and I still passed the first time.
The best part of the BRC is that they supply just about everything: motorcycle, helmet, instructors, materials, etc. You just need to have long sleeves, gloves, and over-the-ankle boots. It’s a great way to see if you even like the feel of riding a motorcycle without making the financial commitment of a purchase.
As rb said, if you can take the class, then go make your purchase, the things you learn there will be fresh in your mind as you start riding.
BTW, when you start looking into taking the BRC, keep in mind that it fills up very quickly, at least around here it does. You may have to schedule it months in advance.
Hope this helps. Best of luck to you.