Gather the parts
Everything ships from Amazon or Adafruit. Total ≈ $130 for a 1 TB build. The Pi 5 is the only non-substitutable part; everything else has good alternatives.
Why an NVMe SSD instead of just using a bigger MicroSD?
MicroSD cards have a limited number of write cycles and are dramatically slower (~50 MB/s vs NVMe's ~500 MB/s). A Pi running a database on an SD card will wear the card out in months. The NVMe sits on a HAT that plugs into the Pi 5's PCIe port — it's the same storage shape laptops use, so a 512 GB drive is under $50 and a 2 TB drive is under $150. The MicroSD on Ugh! Storage only holds the operating system; none of your files ever touch it.
Does the PSU wattage really matter?
Yes — don't reuse a Pi 4 charger. The Pi 5 expects a 5V/5A (27 W) supply. With only 15 W, the Pi still boots but PCIe (the NVMe) gets de-prioritized and may fail to enumerate, or run at reduced speed. The official Raspberry Pi 27 W USB-C PSU is the known-good option.