I tried the On-Hit build, myself, when I was using her on the PBE. It did not really work for me.
I was almost certain it would, as there seems to be a lot of synergy there with items like Malady and such... it just didn't work out as well as expected. I think the main issue was the lack of "tankiness" you typically get from the On-Hit items that work well with her.
I found going kind of a "hybrid" build worked for me, where I would get a RoA or other tanky AP item first, and then go for other "bits and pieces" that work well with her like the Gunblade or some other form of lifesteal, some CDR, perhaps another tanky item, etc. All depending on what I was facing.
As to the question about lane sustain... two things to keep in mind. One is that spider W heals. It is not a "great" heal, but it is usually enough to help sustain in lane if done right. A single lifesteal item can help out here, as well. The second is that the spider form is manaless. This is a huge one. This means you should pretty much ALWAYS farm a minion wave as a spider, unless your opponent is close enough that you can harass as human AND farm. If you farm as spider, you spend no mana and you heal; doing double-time for covering your sustain.