Sculpting Is making multiple head sculpts worth it?

This is entirely up to you and what you want out of the preoject, but here are a few thoughts:

1. How good is your grasp on the process as a whole? Do you intend to do the casting yourself or send it off to a caster and if so, do you intend to take preorders to cover the cost or pay for it all yourself? If preorders, do they need to cover your working time too? If so, do you allow for experimenting and learning?
2. What medium are you working in? If you do digital scultping, then making several variations of the same head to evaluate the potential in different expressions may be a valid way to go. If you work in traditional materials there are options such as making a rough sculpt out of modeling clay and experiment on that before either do a rough cast in some easily finished medium, or use it as three dimentional blueprint for the actual sculpt. It's potentially more work than digital, but doable and still a lot less work than sculpting them from scratch. This assumes that you want several differnt expressions on what is essentially the same face, though. For traditional, this also assumes at least a basic grasp on moldmaking and a larger range of materials. If you want differnt faces that just happens to have different expressions as well, then it may be better to just make a new head.
3. How large a project can you manage at one time? Every extra part you make means many hour of extra work. Even seemingly simple decisions such as between double and single joints makes a clear difference in total hours spent and it can add up fast. If you intend to eventually sell your dolls (though to be realistic, most first doll projects never reach that stage successfully) you need to take the working hours spent and how the dolls will be sold in to consideration as well as it's no longer just a matter of if you are willing to put in the hours, but also about if you will need to ask the buyer of the finished doll to pay for those hours. No simple answers here.
4. What is your priority, to make a full doll that you are happy with or to get something all the way to casting?
If it's the former, then perhaps focus on sculpting the body first and then make one or several heads for it once it's finished to avoid getting overwhelmed. If you decide to cast you can cross that brige when you reach it.
If it's the later, then perhaps make a project with fewer parts, such as just a head and spend all your efforts on making it a really really good head and learn all the steps for finishing, mold making and casting. And if you decide to then send it of to a professional caster you will learn all the steps involved in that as well. Less money at risk, easier to replace parts if a shipping disaster happens etc. And if you intend to sell right away it lets you figure out packaging, marketing, payments, customer service and all that as well.

So, to sum it up: I would advise against trying to do everything at once if this is your very first doll making project. Focus on one goal at the time. If you have previous experience with one or several of the steps involved, however, you could probably get away with more. YMMV.