I'm also planning on producing an m20-based ruleset, so I can share what I understand, which is hopefully correct.
1) Check this thread, which also describes the difference, but the main distinction is between OGL Content (freely available) and Product Identity (owned by the creator.
2) So as you're assembling the content, feel free to include OGL Content. It's maybe good form to attribute the source if you pull directly from OGL Content, but not strictly required? However, the important thing to remember is that OGL Content you use needs to remain OGL Content once you release. You can't use OGL content and say it's proprietary. By the same token, if WotC has labeled their own content as OGL (as they did with their SRD and 5E Basic rules I think), they can't stop you from using it, lawyers or no. Unless they revoke that license at a later time, maybe they can do that...
3) On the other hand, content you create yourself can be labeled as Product Identity. This prevents others from using it as-is in their own products. You may consider specifying items such as e g.. your world's name (e.g. Dragonlance's Krynn), character/monster names (like Drizzt), etc. One important thing to note here is that you can't copyright a concept, only its presentation. So if you come up with the world's greatest slap fight mechanic for your game and call it Product Identity, someone else can describe it with different words and artwork. They just can't use precisely YOUR presentation of it.
4) Lastly, there's nothing wrong (or even "scummy") about planning to sell some work you've created, whether or not its based on existing OGL Content. You're still expending your time and energy to assemble it, edit it, give it nice formatting, maybe some artwork, etc. If you do borrow largely from m20, particularly the Compendium, I might suggest making a contribution to RetroRoleplaying.com if you end up making some dough off it. As I said before, not REQUIRED by the OGL license, but arguably good form.
Maybe some wiser and more experienced authors than I can fact-check me here...