The reviews would suggest 8GB is ok, the M1 uses memory differently to traditional chips (someone will explain that better)
There are lots of reviews on youtube, search video editing or FCPX on M1 Macbook Air/Mini
As for the HDD, Apple upgrades are silly expensive and you are not going to be easily able to get into the thing to add extra. I get away with 256GB SSD in my MBP and on the iMac, but have a 1TB SSD
that I use between the 2 for video and photo projects.
As for buying one, find someone with a .ac.uk or .sch.uk email address for 10% discount via Apple Edu Store