8gb is enough unless you were using more on your Pi already. Using 2gb running btc node and lightning wallet. My Pi uses 4.5gb with much more Apps.

Ubuntu OS needs about 700mb-1gb of RAM.

This might be helpful:

https://www.crosstalksolutions.com/orange-pi-5-simple-overview-and-installation-with-m-2-ssd/

You can run it either as the Pi on SD card with external drive or OrangePi provides a Pcie connector.