Yes, 300k USD in cash will do nicely for your wife and even three or four children. State your income in the tax section by entering the number on the "Total Income" line of your past three tax returns. You will get a notice "suggesting" that you "MAY" want to use a joint sponsor. Just ignore that message. You are a self employed investor, in the present tense, so your job offer is meaningless. If you want to use the income from a future job, go start working now, so you can state the full 70k as your current income. You do not "estimate" but if your salary is 70k that's your current income. (again, only if you are actually working at that job presently)
Presumably you have other assets producing investment income. State and document those assets too. It's time to become an A-Student of the I-864 and it's separate instructions.
You are at NVC. Your wife is still a few months away from a visa.