A thermometer is essential if you don't already have one. (Can use STC-1000 if you have one for fermenting already). A skyhook (pulley) and a cleat to tie the rope off to are the next 2 most important.
My next priority would be marking the sight tube so you know your volume at a glance. I did mine with texta the first time, but that washed off after a few brews. It now had light hacksaw marks every 2 litres, with bigger marks @ 10, 20 and 30 litres.
Scales. For weighing grain and hops. A hopsock (or just use the same bag the grain was in).
And a cold beer or 3.