I've also been trawling through the options ending with ellipses at the end of that drop-down such as Transaction Fields... User Fields... but have still not been able to find either Sales Description or Purchase Description.

Try "Memo".

Resolved by @Justin B here: https://netsuiteprofessionals.slack.com/archives/C298P0BCK/p1676483215206949?thread_ts=1676482830.203319&cid=C298P0BCK