Seems reasonable to me, an ultra is pretty much anything longer than a marathon, so if you can manage a marathon well enough then you should be fine with just a few tweaks to your training.
I don't see the need to sign up for next year already. Don't wish your life away, as my mum used to say. Enjoy the events you are doing this year, focus on them, and see if the desire is still there afterwards, in which case you can spend the winter looking for your ideal first ultra.
There's no rush in the ultra world.
In terms of events, just use the events tab on this site, or Google, or just keep your eye on these forums and you'll find the one for you.