There are actually many different types of fashion mannequins. Some have very limited movement, others a lot. I had a job when in college doing store window displays and the bigger department stores had many different types of mannequins. Some were very detailed. So if you could get one of the better fashion mannequins they can be posed in many different ways.
The standing mannequins that are used for a lot or dresses when the arms show don't work since you can not pose them sitting or bend their arms or legs. The hands are always solid so you can not pose the fingers but believe it or not there are different hands that you can switch around for open or bent hands or hands that can hold objects. But to cover every situation you would need a storeroom full of mannequins heads and hands and wigs -- kind of like Macy's basement.
But in looking at this artist mannequin, although it has movable fingers and toes I do not think I could use it as a guide in painting hands. It is my opinion that a person's hands are every bit as expressive and individual as their face. And there is no life or veins in these wooden hands. I try to capture the individuality of a sitter's hands even in a loose painting.
Also, I would think you would need more than one size mannequin to have it work even as a clothing model. You would need at least one male and one female, never mind children. I question just how useful this would be vs. just getting a look-alike to pose in the same clothing as your subject, especially for what these cost.
I have had stand-ins sit in place of clients that could only pose once and I always looked for someone of similar build and stature. I suppose if most of your clients are adult males then this could work out well but I would have to get a lot of use out if it to justify the cost. I am interested as to how this works out for you if you do buy one. Please post an update after you have had it awhile.