If they do live they will be extremely low and another auto-attack will kill them. You want the sustainability first and some AP before building this. This item gives you Spell-Vamp, Life-Steal, Ap, AD and a cast-able slow.
This is a great replacement for the Runic echo it gives you spell-vamp but the added condition of a life-steal and a cast-able slow is nice.
You can do both and I do change them depending on the enemy team comp. Continue to farm till 6 or do some early ganks to try and get a lane ahead. The reason being is because if you mark someone with your Q it resets your ult when used on someone who has been marked with your Q.
If they are running Skill CC's I want to have a better chance of dodging them or if they have a point an click I want to increase my chances of getting away before they get it off. If you wish to continue to farm head to the krug camp clear them and your raptor camp will have respawned clear those and then go back and buy your items. For those who are new to diana hitting someone with your Q marks them with Moonlight.
The ability will phase through and NOT HIT A SINGLE PERSON.