I'd chuck a cement slurry down first (cement/water mix) to make sure the concrete adheres to the old surface - that might reduce the risk of the joint cracking too.
I'm certainly no expert but it wouldn't worry me about the concrete abutting the plaster.
As mentioned above, the proportions of the mix to be used for the step is important. What height is the step ? it's hard to tell from the pix.
Looks to me like making the shuttering is the hardest part of the job - other than that (and the mix proportions), if it was me, I'd just buy what I needed and go for it.