You can get an older model Samsung Evo 1tb SSD new for under $100. Those have been good drives for me.
You can probably find something to shuck used if you don’t care too much for reliability.
Regardless, get a second disk even something attached to your main PC to handle backups.
RPi are nice, but imo are getting expensive and if you aren’t using the i/o pins just not worth it. If I were to just start out again I’d pickup a used laptop. Higher specs than a RPi and built in battery backup.