I've never found a part for my M-37 that could not be procured, outside of a new hood. As far as brake parts go, every single original piece is available from multiple sources.
The original brakes will do the job. You may have to replace all the parts from end to end now. They are 60 years old. If you want to take shortcuts and save money, you just have to get under the truck, discover the problems, and replace as you go. I ended up just replacing a few lines rather then all of them, a master cylinder, and one wheel brake cylinder and the brakes are appropriate for the vehicle's age and speed now.
No need for disk brakes. It is overkill unless you change out the drive train, which would defeat the purpose of having this rock crawler in the first place. I would not recommend that. Going to work on it, testing, driving it, and reworking your weaknesses after bleeding will tell you what you really want to know about costs, but order a new master and a couple of wheel cylinders for a starter, along with some brake fluid. A closer inspection will tell you if you might need some new lines for a starter, but all are available, from front end to the rear, so don't worry about the parts availability. I would just estimate $400 for a budget for starters, and you might do it for much less. Providing a better estimate is impossible without knowing your real needs, a visual inspection, a test drive, and budgetary constraints, but if you're thinking of disk brakes as a possibility, I would think you could afford a complete new original brake system without any problem. That could be done for less then $600 if I'm not mistaken, which would replace every line in there. It does not include brake drums and shoes. Just the hydraulics. Inspect further for additional expense inside the wheel hubs for shoes and drums needing replacement.
You can see the numbers vary even here in print, due to lack of knowledge of what you have. All we can provide is ballpark based on different philosophies and your inspection.
Good luck with it.
