I think that you are asking whether Photoshop is the correct tool for creating a 3D rendering of a human… Answer: No
Photoshop is a digital compositing or photo manipulation tool and does not do 3D modeling. However, if you already have a model with textures baked and saved you can pull that 3D object into Photoshop CS4 in order to composite an image.
You can draw/paint or composite an exisiting image into another with photoshop - which is probably what you will be doing if you do use Poser, Maya, 3DS Max etc… there are several tutorials out there on how to do this. Thes best sites for digital painting techniques are Ballistic Publishing (tutorials are not free but they are in depth and very thorough), 3dtotal has some tutorials that are free and you might be able to get what you need from there.
I would suggest that if you are just starting out and really have no experience with 3D packages that you try Poser or DAZ 3D. DAZ 3D is free but the add-ons will cost almost as much as buying Poser, either way you can purchase different human forms and outfits to suit your needs. Check out Renderosity.com or ContentParadise.com to view what is available. Once you are done creating your “Human” you will still need Photoshop to do post render finishing and both of those sites contain a wealth of information and tutorials on how to do this.
In the end it doesn’t matter what tools you use or how you do it - the end product is what is important. If you have any amount of drawing capability and know how to render an image by painting it - you may be better off just doing that. 3D packages are complex and take a lot of time to learn. It isn’t instant and you will need time to learn enough to accomplish even menial tasks.
Good luck!