Yunohost currently only packages an old version of Lemmy AFAIK. So until they update it, I would not use it to host Lemmy.
Lemmy is currently more CPU than RAM limited.
80gb storage will last you quite some time as Lemmy currently does not proxy images from remote instances. So only small thumnails and images that users from your instance upload are stored on your server. This might change in the future though, as image-proxying helps with load-balancing a lot.