Can I change my hub so I can put disc brakes on the bike

We have to assume, as you haven't provided a photo, that the frame has the attachment points for disc brakes already -- certain bikes have a provision for rim and disc brakes and yours could be one of them.

It is unlikely that you will find a disc - compatible hub with the same "flange" dimesions as your current ones. This is where the spokes are hooked into the hub, if you don't already know.

Therefore you will need to calculate the correct spoke length for the new hub and order the correct length spokes in order to build the wheel. It is very easy, and often quite cheap, just to buy a complete disc compatible rear wheel either new or used, unless you want the experience of wheelbuilding.

Having them custom built for around $200 is possible if you reuse your rims and you use inexpensive hubs. Spokes will run about $60 for 100 DT Swiss champion 14g silver. I wouldn't re-use your old spokes anyway given that they're, well, old. Making the wheels yourself can get a bit involved in terms of know-how and tool$.

If you're on a budget, look for wheelmaster wheelsets. They're lower end but they fit your needs and are under $150. They're usually laced with 8-10s freehub. See https://www.modernbike.com/wheel-master-26-x-1.5-mach1-sub-zero-disc-mt2000-wheelset

Stay away from the mechanical caliper/lever sets. Hydraulic isn't much more expensive (~$70).