I didn't want to inundate you with links, so I gave only those. You might want to poke around other links on your own.
A different link, not included above, stated that you can contribute up to 25% of an employee's pay to a SEP. For you, that means you can place 25% of your yearly income into a SEP and that income won't be taxed to you until you withdraw it. The first link gives you the percentages you can contribute to a 401K plan.
I think you can borrow against a 401K but cannot borrow against a SEP. If that might be important to you in the future, you should probably nail that down (along with all your other questions) with whomever you discuss this stuff with.