Set
your own goals and desires. Too often I've heard there's
too much in my products after I've already declared how much of a success I find them. I worried about interactions, especially with the EOs, but So far I haven't run into an unwanted consequence.
A couple different EO variety packs and strips of paper to mix scents. Would be cheap, But I haven't used Essential Oils for their scents yet. Small test batches should be pretty easy and cheap. Sunflower Oil apparently carries deepest into the skin (i've heard). Most of your oils are cooked with somewhere for cheap formulation.
Can't help you with scaling up for sales. But That also means I'm not considering "production".
Do a lot of research, even off topic. Some of the answers I needed came with lessons about other topics. You're looking at a body
oil. Balms, salves, creams lotions and soaps all use the same oils. Lots of experience on here