Yes, you definitely want to start at the top. Migrating WSUS to a new server is actually fairly easy. You can find instructions here
. The one difference in the way I usually do it is to backup everything from the old server and shut it down. I then install the new server and name it the same as the old server. It's not necessary (obviously), but it does cut out the step of having to reconfigure your clients.
I guess there is another difference...I don't actually export the metadata from the database either. I just copy the .mdf and .ldf reattach them on the new SQL install. Instructions are here
. Technically those instructions are for moving the database from one drive to another, but the same steps still apply.
As far as the downstream servers, I'm pretty sure you can just install a new one and point it at the upstream master. They should pull all of the groups and settings automatically.
IF you did a completely new one without migrating settings you'll pretty much have a mess on your hands. None of the clients will ever properly check in without removing some registry keys and running "wuauclt /resetauthorizaion"...on every one of them.