It would depend on what you plan to do and your skills in fabricating some of your own stuff using things like pressure sensors, inductive pickups and so on....
One of the most useful accessories is a low amp current clamp. From there a secondary ignition wand and a hi amp clamp may follow.
Once you get your scope you wouldn't have to wait for these accessories to come: just start your diagnostics journey with what you got on your hands til more "stuff" arrives.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
