Depends where you are. Maybe in Canada you need that, but here is Southern California I grew mine in straight water without changing it for months. Pencil trees are like a weed to me now, I can't get rid of them faster than they grow.

Yes, be careful of the sap. You might want to wear glasses of some sort when dealing with it to repot or trim. Consider planting away from where kids and dogs play.

I grew mine in regular potting soil 50/50 with perlite and it did fine.

I agree with Rina, but I have mine in a large rectangular planter in my back yard, as it tends to get quite large. I used cactus/palm soil from HD and have it with a propeller plant. They seem to do well together. When it is by itself, it tends to fall over, at least until it makes a huge clump. Here are some at the Los Angeles County Arboretum:

The stories I've read in the forum regarding this plant, have put me off buying one for myself. It's so beautiful but so dangerous.