I'm a fan of Daz3D myself. As Fleet said, there's a lot of free stuff available to work with.
The basic choice you need to make at this point is Victoria 4 or Genesis. V4 has been around for a number of years and has the most stuff created for her. Genesis is the future and will end up replacing her eventually. Victoria 5 is a Genesis character, for instance.
The free version of Daz3D 4 comes with the basic Genesis character. I believe the base V4 character is also free and will work in both Daz3D 4 and 3. So you might as well get Daz3D 4.
https://www.daz3d.com/i/shop/itemdetails/?item=12194The most important thing you're going to need to buy is Morphs. These are what let you turn the base characters into different people.
If you want to go with V4, the most important set of morphs are called Morphs++ and come with this bundle (which is currently on sale for $25):
https://www.daz3d.com/i/shop/itemdetails/?item=4781If you're going with Genesis, you want the $32 Morph bundle:
https://www.daz3d.com/i/shop/itemdetails/?item=12989Either is a reasonable way to go.
Note, however, that the clothes written for V4 won't necessarily fit on Genesis. So you may be better off with V4 for now. The other option is to buy the Autofit tool, which makes V4 clothing fit on Genesis.
https://www.daz3d.com/i/shop/itemdetails/?item=12666Alternately, you go with Daz3D Advanced, which comes with the AutoFit tool but is pretty expensive.
https://www.daz3d.com/i/shop/itemdetails/?item=13174For the cheapest initial investment, I'd go with the free Daz3D 4 and the V4 complete bundle. Total cost $25 if you get it before the end of the year. Note that, if in the future you want to move to Genesis, there's a $20 tool to import V4 morphs into Genesis. So you really can't go wrong with V4 at the moment.
Tlaero