Gotta say that's a nice rig, I wouldn't do it. Buy a junker...if not to practice on first anyway.
A few reasons why you should not do that.
1. The final Brute is still going to have the axles, engine, transmission, suspension, interior, frame, etc. etc. as it did when you started unless you are prepared to spend a bunch upgrading it. You want to start with a good clean example, not something that needs a lot of fixing up.
2. Do you really want to "practice" with a more than $10k conversion? You aren't going to do it twice. You either take your time and re-do each step until it's perfect, or entrust it to someone who you know can do it well.
3. When you finish building your Brute... you're probably not going to get much use out of that nice standard TJ because you'll take your Brute out every time.
4. Maybe it makes more sense to build the Brute and buy a junker for tougher trails that you'd be worried about with the Brute's wheelbase / departure / expensive bodywork.