I'd definitely do backups, but the easiest way to get the databases back online will be detach, re-attach.

On the system databases, it kind of depends on what you've got in them. The master database is probably fine to use from the re-install. That's usually less complicated than trying to recover the old version. But it depends on whether you've made changes to it. For example, are there any procs in it that have been set up for auto-run on startup?

With the msdb database, do you have scheduled jobs? If so, restoring is probably going to be easier than re-creating those.

On model, have you customized it to make your new databases have certain properties and/or objects?

With tempdb, of course, the main question is what size works best for it. You probably aren't backing that one up anyway, but you'll want to make sure the new one is the right size, which you can probably figure out from the existing one.