Building Singed anything like a tank is just doing it wrong..
I usually do the same as you, but changing hourglass with Force of Nature, but.. pretty much the same :D

Why build caster? why?

Singed's job isnt about killing, you're a tank

You can't rely on enemies chasing you throw your poison. If they do, they're retarted anyway and slowing them a bit won't hurt. For offensive use I think the slow is actually pretty good, even though Singed of course has his own slow ( Mega Adhesive).

Overall, however, I think building Singed fulltank is much better. It will allow you to simply run into the enemy team, completely disrupt their AD carry (making them pretty much useless in teamfights) and zone them so your team can finish off the rest.
You can deal quite some damage by building AP but that's not really your job. I would also suggest using 3x Greater Quintessence of Movement Speed instead of your 3x Greater Quintessence of Experience (which is pretty much useless).

Also, if you're laning top, get Teleport instead of Cleanse, simply use your ultimate to reduce crowd control effects on you

Nice guide and all but you should never get rylai's on singed. Your supposed to want enemies to chase you, not slow them.

Wouldn't you rather get Movement speed Quints instead of experience?

You forgot fling for lvl10 :) Jz an ok build :)