Nice project!

Given the tight budget, here is what I would do, especially if you are not too constraint by space and don’t mind a few extra watts of power consumption. The Raspberry Pi are getting expensive, and the 200USD will barely get you a RPi5 nowadays. You said there is no market for you for second hand sever hw, but I’m guessing it should not be too hard to get used office desktop PCs.

• Get two of these. Maybe Optiplex or Thinkstation. You can probably get something decent <50USD each.
• Get two small SATA SSDs for the OS (128 - 256GB), around 30USD each.
• Get your storage drives. You should get 3 of them so you can have ZFS raid redundancy.
• On one machine, install TrueNAS and your storage drives. Default RAM is probably enough.
• On the other one, upgrade the RAM to 8 or 16 GB (~50USD), install your favorite Linux distro, and you can run your services, accessing the storage with NFS!

To me it feels safer (against my own mess-up) to separate the storage and the services, plus this setup is fairly upgradable. You’ll probably have space to add more storage drives, even maybe a cache SSD; increase the RAM; add a third machine etc.

Of course it’s just one idea, maybe other another layout might fit your use-case better, idk.

Good luck!