There are other factors to consider as well before deciding on SEP IRA or Solos 401(k). A SEP IRA does give you flexibility in contributing one year and not contributing in another, you cannot take a loan from the SEP IRA. You will also have no Roth option with a SEP IRA. Solo 401(k) is only there if you are the sole employee of the company.
You can easily open and manage a SEP IRA yourself. Most brokerage firms will handle the back end forms and requirements while you manage the investments, like a regular IRA. Whether you use someone to manage the investments depends on your comfort and knowledge of the investing.
The contributions do not have to be made quarterly or throughout the year. They can be made at the year end or by April 15th tax filing deadline to be deductible from your tax return. You can also make them in small chunks during the year if you like. I would recommend making the complete contribution before filing the tax return.
