DHG doesn't NEED co2, it just grows really slowly without it.
Many people dose with Flourish and Flourish Excel, and find that suffices without the extra complexity of co2.
The Hagen system you linked works great, by all the reports I've read. I recently did a lot of research, and I nearly picked it up myself but decided to stick with Flourish and Flourish Excel for a while longer before I go co2, for a few particular reasons (that won't be an issue for you).
The Hagen system is just a fancy store-built DIY system. You don't have to jury-rig gaterade bottles and epoxy airlines. It's a really nice, if somewhat small, DIY container that you're paying for. The ladder is huge, and many people end up switching to a regular diffuser instead. The pre-mixed mixtures are really overpriced... most people don't bother with them, or at least just use up the first included batch and then move to simple recipes. There are many listed in the Amazon reviews, really easy to pick the one you like, just add the right amounts of baking soda, yeast, sugar and warm water and that's it (same as any DIY bottles).
In short, if you have the spare $30 (I saw the same kit at Amazon a week ago for $22, so the price fluctuates, just FYI), it's a very low-hassle way to get into DIY co2. If the $30 (or $22, if you're patient) seems a bit high just for a nicer, more attractive DIY bottle, just go with the usual gatorade/water bottle method.