Hello and welcome to the NodeBB community!
Are you using nodebb-plugin-import?
That seems like the only one worth a shot today from what i can find.
Simply follow the instructions in the README, there are some handy tips there for converting BBcode and such aswell.
Sure having a separate database server is recommended, but really not needed - you should backup your dumps externally just in case if you don't though.
If you're going with redis, this may be useful for that cause.
This probably isn'tthe most helpful post, as i haven't migrated from another software myself, but i'm trying to help in possible ways.