Changing paths is okay if you can explain it well, but bad if you cannot. You do not want to come off as flighty, a degree collector, someone who hasn't decided what they want to do in life etc. It's not totally clear to me that you cannot get to your goals by merely doing psychology/anthropology and skipping the med school aspects, so I would think long and hard about that. Med school is about training folks to be clinicians, to take care of patients, so that is what you really have to want to do, and what you will be expected to sell yourself as interested to do if you want admission to med school. Med school seats are limited and the mission of all med schools is to train physicians to help address healthcare demands for patient care. If you come off with goals of being an academic and not a clinician, many med schools are going to lose interest fast.
As for having a very specific medicine career goal, it's generally a mistake. Most (>80%) of all folks in med school will change their mind as to specialty at least once in med school. It's fine to have leanings when you enter med school, but a bad idea to be locked into a path, particularly one that is necessary because it's combined with a PhD that requires a particular specialty to make sense in order to use both degrees.
From personal observation psych is not an exception to the "change your mind" concern -- I'd say most of the folks I know who started out med school planning on psych ceased to be interested in psych by 4th year. It's one of those fields that is somewhat different in practice than on paper, and so while some folks really enjoy it, many others cannot see themselves practicing in that field or find other specialties that seem more appealing or better suited to their personalities. So unless your anthro combo works equally well with, say peds, OBGYN, path, derm, etc (whatever you ultimately like best), I'd say it's a huge mistake carving out such a niche goal before the vantage point of 3rd year rotations.
The first thing you need to do is start shadowing physicians, and volunteering in local hospitals. You do seem totally "in the blue" notwithstanding your last sentence of your post -- non-sci majors frequently are and nothing in your post suggests you are particularly well keyed in as to what the practice of medicine in the US in this current day is like. This is not a decision you want to make in a vacuum -- it's a very long and hard path, and not one you can easilly or inexpensively extricate yourself from. My concern from your post is that you seem focused on academia and not on patient care; though you may end up there, you need to sell yourself as being interested in the latter, because patient care is the focus of med schools. So get some exposure to patients and doctors interactions with them. It will be required for application anyway, but I think you probably need it because I worry from your post that your interests don't really lie in this area. Based on the fact that you are coming from another field altogether, also want a PhD, and want it in anthropology, which to me suggests a professional distancing of yourself from patients that might be a bit opposed to the up close and personal focus expected in medicine, and is not a health science that readily lends itself to industry or is easily combined with a clinical practice.
Let me ask you this -- if you couldn't do a PhD, would you still be interested in medicine (and clinical, not academic medicine), and why? Because generally this needs to be the driving force here. You go to med school because you want to be a doctor, period. If the doctor part is simply a nice add-on to a largely non-medical career, you are making a mis-step. If the primary goal is the academics/PhD part, then get your PhD, perhaps combine psychology and anthropology in some fashion, and leave medicine out of it. If, however, the primary goal is being a physician, you have to think long and hard as to whether the anthropology makes sense in the event you decide clinical psychiatry isn't what you actually find you like in practice. And also bear in mind that if you find you want to be a practicing eg surgeon/primary care doc/ophthalmologist/etc, the PhD is going to add a lot of years to an already long path and perhaps add nothing to your career because it is so far afield from healthcare.
As for where you do your prereqs, Iowa is fine.
Again, I'm not saying you cannot do what you plan. I'm just saying that currently your game plan sounds awfully half baked, and doing a lot of shadowing, volunteering and having a much stronger and clearer focus on the patient care aspects is going to be hugely important. Most people don't end up academics. And as a non-sci person who has yet to take the prereqs, carving out a very specific academia path might not be as advisable as a path more akin to simply doing well in the prereqs and MCAT in hopes of "getting into med school". You need to flesh a lot of stuff out and I think it starts by getting yourself into a hospital and seeing what doctors (who are not relatives, and who practice in the US) actually do on a daily basis, and deciding if that's what you really want to do (and why).