It depends on what you want to do. I just started this last February, and I thought I was going to use V4 and M4, so I got the basic stuff for them (morph++, elite morphs, muscle morphs as the sales came along) but I’ve found I rarely use them. They’re handy to have for using GenX, which transfers older characters over to Genesis. The older characters are often cheaper, but that’s the only thing I use them for. That, and using Posemaster to transfer good V4 and M4 poses over to Genesis.
Genesis is for me so much easier to use, and it bends much better. I think there are bending fixes you have to buy for V4 and M4. So I have to admit that if you aren’t already attached to V4 and M4, I would go the Genesis route. Most clothing transfers over very well, except for some shoes and long dresses / long cloaks / long capes.
From what I can tell, It costs more to use the 4th generation figures because you often have to buy unimesh fits if you want to use one piece of clothing on a different model than it was designed for. With Genesis, clothes designed for all Genesis shapes (Hiro 5, Stephanie 5, M5, V5, Hitomi, etc.) will work for every other Genesis shape without a hitch. Plus it’s quite easy to design your own unique characters with all the shaping dials Genesis has.
My first recommendation is to buy the old Figures, Avatars, and Characters book, first edition. It comes with a bunch of older content that works with Genesis. (It comes with V4, A4, H4, and M4, their starter packs, the 3D starter pack, Apartment 39, a bunch of animals, etc.) You can find it here. It’s well worth the price!
My second recommendation is to get at least 1 month’s membership in the platinum club if you’re planning on buying a lot. You’ll get access to thousands of $2 items, which is well worth it. But you’ll also get 70% off all DAZ originals for the rest of this sale, instead of 50%. And once this weekend sale goes off, you’ll get 40% for the rest of the month. In December it will go back to normal, 30% off DAZ Originals. If you calculate it out, you’ll probably end up saving a good amount of money going this way.
If you go the genesis route, but you still want access to the variety of Gen4 characters made for V4, M4, etc., then you’ll want to get morphs++ for V4 and M4. And then you’ll want to buy GenX when it goes on sale so you can transfer the characters to Genesis. If you want, you can get the elite and muscularity morphs, but they are used far less often than morphs++. Also, as a side note, the V4 and M4 elite texture bundles are pretty high quality for closeups, so those can be handy. But it’s not a crucial thing. You can pick those up in other sales. They work great on Genesis, although apparently the eyebrow shape changes slightly from what others have said.
The next thing I would buy is the Genesis evolution head and body morphs. (Edited to add: Genesis Evolution Muscularity is helpful for adding realism into renders, since there’s a function you can turn on that automatically flexes the appropriate muscles when Genesis moves.) The evolution expression morphs are handy, too.
Then, to optimize autofit, I’d buy all the Gen3 and Gen4 shapes for Genesis. David 3 for Genesis is sold separately, I think. These aren’t just the shapes, they also include UV maps that allow autofit to transfer clothing made for these characters as well. And yes, do get the V4 and M4 shapes for Genesis, too. I didn’t at first, since I already had the UV maps (they come with DAZ3D 4.5 and I had the original V4 and M4. No need for the V4 and M4 shapes for Genesis, right? Wrong. 
Some characters for Genesis use the V4 and M4 shapes, and the character won’t load properly without them.
Then there’s the question of whether or not to get V5, M5, S5, and H5. Essentially, they’re shapes for Genesis. So all the clothes and hair that come in their packages will work on any genesis shape. But they come with their own UV map, which means the characters created for them (the shapes and skins) will only work with that UV map. So if you want the characters based on the Gen5 shapes or their skin textures, then you’ll want to buy the base shape. I have ended up doing that but you may decide that one of the Gen5 shapes just doesn’t suit you.
The other thing that I have found incredibly useful is Marieah’s Fabricator and the add-on packs that go with it (Ye Old Clothes, Lacemaker, Totally Bazaar, and Machpack.) I love, love, love it, and her store is 50% off right now. These are shaders that allow you to apply new cloth weaves and colors/patterns to any clothing you have. It’s beyond awesome. That means that older items with lower resolution textures can be repurposed and become entirely new. Plus there are a bunch of free add-ons at sharecg for this as well. This is great for making unique renders that you can personalize to your taste. Here’s the link to the old forum thread that had the freebie links and a discussion on how to use and render with these shaders. And don’t forget to take a look at her Gemologica, as well. It does the same thing as the Fabricator, but for metals and gems. The possibilities are endless there, and very, very fun.
There’s a good number of other helpful things in regards to lighting, posing, etc, that I’ve found, but none of them are on sale right now, and none of them are essential to getting started.
Also, keep in mind that there’s probably going to be a Christmas sale of some sort, and March Madness has a lot of artists’ stores half off, as well.
