With any NUC of the 5th or 6th Gen with an i5 CPU, you’ll be covered for running Roonserver. If you like to overspend, you could go for an i7, but consensus says it’s not really necessary. Run Linux if you can – there’s a good guide for doing a minimal Arch Linux install here, but you may want to take it easy and turn to Ubuntu Server for an easier install. With Linux, 4GB RAM should be enough, but since RAM is cheap, spring for 8GB or more. Put in a small SSD for OS and database – an NVME M.2 stick would give excellent performance (see this thread for more info).
Migrating the database is quite easy – just follow the outline in this this article.
No, you won’t need a second license. Once you’ve migrated the database, you can unauthorise the old inserter stall upon configuring your new Roonserver. Your active license will be transferred automatically.