With respect, I don't know if you are a new pro starting out, or lifestyle with just one special friend. A pro would naturally want to have a lot more tools. . .
But, either way, I would think a good sturdy set of leather cuffs is a must. Next a small collection of CP tools: at least one paddle, a whip, and at least one riding crop (hint: you can get nicer crops for a cheaper price, I think, from equestrian suppliers than from kink shops).
A blindfold would be nice. Also, I'm suprised how few doms use earplugs. You can get cheap disposable ones from sporting goods stores. A blindfold plus earplugs makes for instant sensory deprivation for your slave, with no need for an elaborate chamber dedicated to that purpose. Restrain him (or her?) when he/she is blindfolded and earplugged and you have a truly helpless and clueless slave!
I personally LOVE nipple "torture" (it ain't torture to me ). You can get the job done with clothespins, but a nice set of clamps -- especialy with an attached chain -- I think works better and looks hotter.