Kassadin ad-offtank not common, but with his ulti, you can dash in and out of combat, which can make him also great at roaming and help other lanes in your team.
In general, the early should be not so easy, until the mid game, where you can deal huge amounts of damage, so try to farm as much as you can and wait for the jungler...
The great advantage of Kassadin AD-OFFTANK, is that you nearly only need to boost his defence early since you can deal a good amount of damage with your second ability. And while you will be AD, your second ability will deal a good amount of magic damage, so the enemy champ cant so easily counter your type.
The main counters are darius, renekton, rengar and kha'zix in early game, so let them push, and when the jugnler comes you can have an easy kill, with any other top, the early should be easier.

Even if you choose to turn him into a tank, his second ability will still allow him to stand for a fight, randuin's omen and your slow will allow you to heavily slow the enemy team if you teleport right in to catch the carry, which should help your team, and the guardian angel and your ulti (and your flash if available) will allow you go get away very safely after being killed.