You can make a Sec. 105(b) (aka healthcare reimbursement arrangement) work... and you can use a solo 401(k) to bump up your tax-deferred savings at lower income levels. But I'm not sure I see an easy way to combine them so you get synergy.
We also don't have any detailed info on your situation's specifics...
That said, here's some stuff to ponder... with a solo 401(k), you can do both the 25% match and then also an elective deferral (so another $18K).
Example #1: With $100K in profits all treated as self employment income for your spouse, spouse could do a $20K "employer match" and then also an $18K elective deferral. That gets you to a $38K 401(k) contribution..
Example #2: With $100K in profits split between spouses (say $32K to one spouse as wages and $68K to other spouse as self-employment earnings), each spouse could do an $18K elective deferral... and then on the employed spouse, business could do an $8K 'employer match'... and then on the other spouse probably about a $12K 'employer match.' That puts you to roughly $56K. Roughly...
Bottomline: You can save a ton into your 401(k)s without having to fiddle with a Sec. 105(b) plan.
Note: I used $100K in income just to make the math easy. Most households make half that amount...
BTW you possibly can do a Sec. 105(b) plan and save income and payroll taxes via that gambit. But it doesn't really jack your 401(k).And if you're thinking that way, you might want to just look at the S corporation option since that'll let you save a ton of payroll taxes.
