It's difficult to compare all of those things. When it comes to co2/light/nutrients, it's 33%/33%/33%...they all have to be in balance. How you fulfill those requirements is up to you. With a low light/low tech setup, the style of filter(cannister or HOB) becomes less important but the amount of filtration(volume filtered, or flow) becomes more important.
If you can't afford everything all at once, here are some recommendations:
Substrate: Substrate is difficult to change once a tank is up and running....difficult, but not impossible, just requires a day of work and a few buckets. I've done it. For this reason, many people suggest upgrading substrate first, but if you don't mind a little extra work, it's less important. Choosing a substrate is up to you. In a high tech tank, you can go with an inert, but you'll need to add root tabs. This can be a cheaper option, but you may also decide to go with something like eco complete or aquasoil. The choice is yours and, like I said, these can be added any time.
Filter:If you are wanting to go high tech, a good cannister filter is very important to avoid off gassing of co2 due to an HOB filter...however, I know plenty of people that run a medium light tank with DIY CO2 that use an HOB.
CO2: This is the thing I would add first as long as I already had a cannister filter. Adding co2 without adding lighting or nutrients won't cause an algae bloom, so, no worries. It may waste a little money(unused co2) but not enough to worry.
You can run co2 with an HOB and add a cannister later, but before going high tech, you should have co2 and a cannister filter. Which one you do first is up to you.
Light: After you add co2 you can add light. 4 wpg, 2 wpg, whatever, as long as you have the ability to add enough co2, you're golden.
Nutrients:If you have co2 and light, the only problem you'll encounter is stunted growth. This will be corrected with the addition of nutrients. These can come last.