Hi Nessmuck,
I don't work in braintanned, so you'll have to rely on other skilled makers' advice.
As far as "just some simple tools needed", I would offer that you need to think about your goal. If you want to make one primitive bag, then a few simple tools may suffice. If your goal is to eventually equal James Rogers' work, I would suggest buying some high-quality, strategically selected tools. Like any craft, buying cheap tools just means a poor product and having to buy better tools in the future. Cheap tools are not worth the money.
I just finished bag #51, and am still on a learning curve. I was just shaking my head this week at the pile of tools I have in three drawers, each tool suiting a specific purpose.
Also, I know some folks are looking for a "quick" or "one night" fun project. Nothing wrong with that, but nice leatherwork takes time. Dyes, oils and finishes need dispersing and curing times....overnight usually for me at each step. Hand stitching takes a long time too. Think through the architecture you want and the steps needed to get to the end before cutting leather. Drafting skills help when making patterns (all of my bags have a unique pattern from poster paper).
I liken bag making to racquetball and bass playing. It is deceptively easy to get your toes in the water, but one needs a lot of diligent practice and experimenting to get good at it. I was blessed that my dad worked in leather for many years, and we sold nice leather as a hobby for 32 years at gun shows. At least I knew what good looked like. As was suggested above, you need to know what good work looks like, and then figure out how to get there. I am also thankful to James and others for generous suggestions over the years.
From my experience selling holsters, one thing I will not tolerate is a bag finish that transfers dye and oil to someone's expensive reenacting and living history clothes. I look at a lot of bags folks have for sale, and think, "I bet they never considered dye and oil transfer when the buyer sweats on it or gets rained on." That is why the back of my straps are not dyed dark, and they are sealed,
One other thing, you cannot make a silk purse out of a sow's ear. Poor imported leather will result in frustration and a poor product. Tandy is now importing most of their leather, and while it looks good, it made me tear my hair out trying to get finish to take. Only buy US hides. Zack White's in Ramseur, NC is good, but make sure and ask for US hides only. Wickett and Craig hides are wonderful, but have a price that suits the quality.
Below is my first ever leather project - Bag #1. Below that are two more recent bags - a haversack and a "worn-in" shooting pouch. Between those is a lot of frustration, gritting of teeth, and sticking to it. Practice practice practice and ask questions. Like I said, I am a long way from "good" and am still learning.
I am sorry this got so long winded. I hope it helps a little. God Bless, Marc