I would also lean on the side of the 35 zombie unit. It states that the converted Chaos Cultists cannot purchase options. But Chaos Cultists don't purchase more Chaos Cultists. The unit can buy extra Cultists. The way I read it, it is worded as one being a unit thing, and the purchasing options being a 'per model' thing.
On the other hand, that's quite a cheap unit for what it can do.