The answer is, it depends.
What style are you interested in? Realistic, anime, toon…?
Are you interested in fantasy, sci-fi, modern, period or something else?
What is the goal for your art? For example, will you be creating professional illustrations, hobby art, a comic strip, a graphic novel, animations….?
Is there a reason you want to go with Aiko 3 and Hiro 3? As much as I love them (and still use them), don’t expect the third generation characters to get much love in the future. Even the fourth generation (Aiko 4 and Hiro 4) are being neglected in favor of Genesis (gen 5.) The big advantage of Genesis is that the clothes you buy can be shared between characters. For example, Aiko 5 and Hiro 5 can wear the same outfits. That can save huge $$$.
Will you be using mostly Daz Studio or Poser? If you decide to use Genesis, it works best with Daz Studio. Not all genesis products will work in Poser.
Will your art focus on just the characters or will you need lots of environment items, too?
Are you a platinum club member? Members have access to thousands of $1.99 items. You might want to browse through the PC items to see if it is worthwhile for you to join. http://www.daz3d.com/platinum-club-items
To get started you might want to buy a few items at a time and fill in with freebies. In addition to the freebies you can find in the Daz forums, there are lots of freebies at places like ShareCG and Renderosity.
You might also look through some of the galleries and forum threads for inspiration. Unfortunately, most of the Aiko 3 and Hiro 3 stuff is in the old archived forums which are kind of hard to browse through.
Once you provide a bit more info I’m sure you’ll get lots of suggestions, such as some bundles that might work for you. Good luck and happy rendering!