Couple of questions

Hi, I'm not too experienced with plants, but I'll try to answer some of your questions.

You will probably need CO2 only if you have more than 2 wpg. That is the time when they will need more co2 to photosynthesis.

In my experience, gravel is an okay substrate. I just use gravel, and my plants are doing fine, though I only have low maintenance plants, like amazon sword plants and anacharis. If you want to keep the more difficult plants, substrate is always welcome by the plants. I don't know anything about substrates, so other members will help you with that.

Easier plants may include but are not limited to: anacharis, java fern, java moss, (anything with fern in it basically ), annubias, cryptocorynes, amazon sword plants (in my experience, are very hardy), water sprite, hygros, etc. CHeck out www.plantgeek.net for a bunch of hardy plants.

And light is probably the most important thing. What is your WPG currently?

Hopes this helps.

My advice is to read lots before you dive in. There's a great sticky on fertilization, and I'm sure there's lots of info on lighting planted tanks if you search the old posts.
Gravel can work, it's a good idea to put some peat and/or mulm beneath it for the plants if you can.
The biggest thing to consider is lighting. You'll need at least 1.5W per gallon. Over 2WPG I'd inject CO2 - light, nutrients and CO2 need to be balanced or you can get horrid algae blooms.
Planted tanks can be high maintenance or lower maintenance depending on your lighting and plant choices, it's really up to you. I made the switch to real plants not so long ago and absolutely love them, hopefully you will too