Welcome, Vale :)

First of all, I'd second what Acadia said.

I use an external runtime (meaning it's not in the Poser folder) as do most of the old timers here. :)
The Daz stuff installs into the right place, but a lot of the free stuff doesn't, so if you unzip something, I'd suggest making (at the very least) a "PoserExtracts" folder and install things into there.
You can add this folder in Poser (you can add any runtime folder to your library) and try things out, without messing up your poser install.

Now, as for realism... that takes a little time. :) You can get some very good results with high end textures and figures, but as with everything worthwhile... good things take time. And practice. And good lighting... and... and...and... :)

When you come to do a final render, don't forget to push your render settings of the Firefly renderer from Draft to a higher setting, or it won't look the way you expect it. (Draft looks fuzzy and unsharp)

I'd recommend joining the Platinum Club at Daz for at least a month and pick up the cheaper goodies that way to get you started. Don't overdo it to start with, get a good texture and perhaps play with that for a while until you are more comfortable. :)

Silke