If your husband opens a SEP IRA he will have a retirement plan at work, so at his income he'll be able to make roth, but not traditional IRA contributions.
You should still be able to make deductible traditional IRA contributions, but you could be close (modified AGI of $183k. See:
https://www.irs.gov/Retirement-Plans/2015-IRA-Deduction-Limits-Effect-of-Modified-AGI-on-Deduction-if-You-Are-NOT-Covered-by-a-Retirement-Plan-at-Work) depending on how much he puts in the SEP.
A better option than the SEP would be a solo 401k. As long as your husbands business consists only of owners and spouses (no other employees) he can go that route and it has higher contribution limits at lower income. At $180K net income from the business he could almost hit the absolute max on a solo 401k ($53k) at $52048, while a SEP would be limited to $34048 (see
http://www.bankrate.com/calculators/retirement/self-employed-401-k-calculator.aspx). The other advantage is that unlike a SEP, a solo 401k won't mess up a backdoor roth IRA if you go over the limit for contributing directly to a Roth.
If you will legitimately be working in the business it would absolutely be a good idea to pay you for your work and use that income to also contribute to your own account in the solo 401k (despite the "solo" you should be fine since you're a spouse). three things about that:
1) make sure whatever you are paid is defensible for the work you do (what would an unrelated person be paid for that work), and
2) realize that since you will be taking some of his income that is currently over the $118.5k max on the social security part of self employment tax (12.4% I think?), you'll have to pay that while he would not, so make sure it's worth it in terms of getting that extra tax deferred space.
3) If you get in on the solo 401k you're also then covered by a retirement plan at work, so you'll both be limited to Roth IRAs. Still definitely worth it though.
You should still do the math on the tax savings vs extra self employment tax, but I think I would shoot for giving you about $19.5k of net self employment income as that would let you max out the $18k employee contribution on the solo 401k, but not much profit sharing which only lets you put in about 20% of your income. By my math if you're making $19.5k and your husband makes 160.5k then your total solo 401k contribution max would hit $66.3k. Ain't self employment great?!
*Edit* Another thing: if you're covered by an employer plan at any point during the year, I'm pretty sure it counts as being covered for the whole year, so you're probably limited to Roth for this year either way. Also, if you do get in on the solo 401k any contributions you make to your current employer's plan will count against the $18k limit, so keep that in mind too.
