I personally own one SR1, I've also got plenty of accessories, some of them is useful, but not everything is necessary
If you want to do slow-motion or under-crank (i.e. to shoot something else than 24 or 25 fps) you need Variable Speed Unit
(made by Arri), known as VSU - it's small controller with rotary knob to change fps, but isn't very accurate.
I'd rather use Precision Speed Control
by Cinematography eletronics. That's really a good controller and costs usually about 250 USD.Viewfinder extension
is something you can really live without.
But if you want to shoot sync-sound you'd need Barney
for isolating sound of camera.
And classic sound equipment:
Zoom h4n or Tascam DR-40 field recorder
Rode NTG-3 or Sennheiser MKH-416 shotgun microphone
Sennheiser ew112p G3 - wireless
transciever / receiver - lavalier microphone set
Rode Blimp - zeppelin for micBoom pole
and the most important - good field audio mixer
- Sound devices MixPre D / 302