Guessing that this would have the air over hydraulic brakes similar to the larger Houghs. No brakes could be any one of a few things.
1 No air, bad compressor
2 Problem with air valves and or tanks
3 Bad master cylinder or wheel cylinders
4 Leaks in lines from master to wheel cylinders
5 Mechanical problem between wheel cylinders and brake shoes, many of that vintage have "wedge brakes" that can be problematic.
6 Brake shoes/linings bad or missing
7 Drums worn beyond limits
Or any combinations of the above or something else altogether!
A lot will have to do with your personal mechanical abilities and the ability to ask questions and follow suggestions. That last part can be a major short coming of too many people!
Air compressor or any air valve problems should not be a big deal to fix, not saying cheap but parts should be easy to find with a little effort.
Problems with the hydraulic side of things might be a little more work to find parts but as DMiller suggests checking with that outfit or others might be a good idea, heck even a NAPA store might be able to match up the rubber bits!
If wedge brakes that outfit might be a good starting place or else find the oldest truck brake shop in the area and the oldest guy there and you might get lucky. Heck I probably tossed out enough parts to do all four wheels over the years.
Not having seen it myself I can't be sure, but would think drums, shoes and linings would be the most expensive but also the least likely to need complete replacement. Again that old guy in the old brake shop is the best place to see about linings.
Just thought of another place you could check with on parts, they don't show Houghs any more on their web site but they were the "go to" place in the northeast "back in the day" for used and aftermarket Hough parts and might still have some or know where to look:
https://www.hrparts.com/
As for price that's a tough one! One thing that I would want to look at is if this is for a single project or something you are going to be wanting to have for years to come. If just for a one time job moving a bunch of material it might be better to spend that $4,000 to either rent a machine or hire someone to do the job.
One could easily spend close to that much more if there is something major wrong with the brakes and once they are working the rest of the machine is, I'm guessing, about 40 years old or more! A single hose failure might might cost $100 for the hose and $40.00 for five gallons of oil, or even more.
I know I'm not really answering your questions but just trying to think up things that only you can really answer. That said I would at the least see how low he will go on the price, you can even print this out to show him on the chance it might help.