2 ranks on E most often outruns all slows in the game by that level. (Max speed for 4.5 secs, no slow exept some last longer early-midgame)
W is amazing spell, and DS' kit revolves around it (exept his wall, unless you're planning on casting shell on the illusions), it's like radiance on steroids with lower radius. Use it on dudes who are stayng next to enemies anyways, and surge them too.
My itembuild goes: start with Soulring recipe, branches and salve/tango, maybe clarities.
Buy Soulring from sideshop, along with normal boots. Get vanguard if you don't seem to stay alive, you should have no problem farming due to W.
After that you go straight for BoT, they're good on DS. Phase is good, but it feels meh to me compared to BoT.
After BoT go wtf you ever wanted, DS is a farming machine, he can jungle, go to lane. Basically you can farm anywhere and anytime if you got BoT.
Good items for example are
Basically a very versatile hero, there's a reason he's top pick/ban right now in Comp-Dota.