I've enter from both Nong Khai and Mukdahan around 10 time each since 2010, on either tourist visa or visa exempt entry (quick bounce), and it was never an issue. Most friendly border to enter. Nothing is ever asked, no proof of, nothing. I never ever showed any of the "3 proof of", this is usually reserved to air entry. What
is describing by land is probably only true for certain nationalities that get quizzed often, but for G7 countries like me (Canadian), and other rich nations, you don't get ask anything by land unless they want to fuck you over in Poipet, for example.
I've been here as long as you buddy, you are not talking to a typical 2 weeks traveler newb. Some people don't meet all requirements so they go under the table with an agent and it will be costly.
I've never heard of cooking for ed visa. CM is famous for self-defense and muay Thai ed visa. Learning a language is also common for ed visa. If you are south Thailand there are other options like scuba diving. Price will be at least 25k baht on the very low end for any type of ed visa, but realistically 30-35k baht. If you use an agent expect a lot more total: 50-60k baht.
I strongly suggest that you research in advance or you will end up like thousand of people with not enough time left to switch to a long term visa. You do not need to pay 30-40k baht for an ed visa with an agent if you truly want an "education" on learning Thai, self-defense, or something else. An agent is mostly for people who want a visa without actually going to class to learn anything, volunteer, etc.