Minimum Income Requirement for Spouse Visa

I am a UK cit and my wife is Argentinian. I will be £2,500 short of the £18,600 mark when I get my pensions sorted later this year. I hope to use self-employment to meet the gap. This will be as a private tutor and largely cash based. Will this cause a problem for the application and how long do I need to have been self-employed for?
I'm unsure also how long I need to have been in receipt of the pensions. My occupational one has been paid for 20 years, but how long do I need to be in receipt of my State Pension before I can add that?

For self employed income, HO would expect to see 12 months worth of income and evidence of Self Assessment and taxes paid to HMRC.