The more layers you add to something, the more unnecessarily clunky it becomes. So you use MP to purchase ingredients. Great. Then you HQ the sub-components. Great. Now you need to combine them to produce different effects. What is this, a crafting job?

Keep it simple. There are two commonly described Chemists, one from FFT and one from BD (Salve-maker). If you want a 'Mix' to be the job's main theme, then just have the potions combo off each other when you use them. We've done combo-based jobs before, right?

I don't personally care about MP. You can have a combo-based potion healer where each potion has an MP cost, but that's a marginal step away from WHM. What I really want to see is a healer who gains additional healing output from doing damage and vice versa. A situation in which throwing offensive items restocks your healing items and throwing healing items restocks your offensive items. You can keep an MP safety blanket in there if it makes you feel better. Just make sure that the bulk of the job plays differently from WHM, please.