Poses do not require the morphs (assuming that you mean posing as in moving the figure to a new stance, such as sitting or running).

As mentioned, not having the morph packs limits the degree of character faces and body types you can use from merchants and freebies. Some merchants sell characters that do not require having the morphs packs, but you'll have to read the product description carefully to be sure. A few freebies similarly do not require the morph packs, but most do require owning them.

Poser 7 has a morphing tool that allows creating your own morphs directly in the application. Magnets are another means to alter figures in Poser. You can create your own morphs in other 3D applications. This means that you do not need the official morph packs to sculpt your own characters. However, there is some convenience to having sets of morphs already designed to make the most common changes to the figure and to apply wholesale changes to the body (such as thin or muscular). There is a certain satisfaction to creating your own characters.

You've got a lot of options to consider including, if you decide to buy the morph packs, whether it starts becoming cheaper to join the DAZ Platinum Club to get a 30% discount on the packs. Another consideration is that the newest version 4 set of Millennium figures are also coming out and, if Victoria 4 is an indication, morphs will be sold in one pack instead of as separate head and body packs. It will be years before there is as much free content for V4 as for V3 and the V3 model is still a very useful figure.