To help, I used Excel to forecast the future value (FV) of both options: 1) leave existing traditional rollover IRA in-place or 2) liquidate the IRA, paying the 10% penalty and 33% marginal Federal tax hit.I'm 15-16 years away from retirement and computed the FV of both options over that timeframe.

Account Closedbetter to rollover 401k to Solo401k rather than an IRA.

You piecemeal rollover the funds into a Roth IRA for some level of tax efficiency. 

If I set up Solo 401, can I still have my existing SEP IRA, or do I have to do roll-over.

I have funded a corporation from my 401k using the ROBS process (Rollover as Business Start-up). 

This could come through contributions or perhaps rollovers as applicable.  

My husband is leaving his old job so we’re looking to rollover his retirement.

Once you leave...game on for the roll over.

I do have a rollover IRA from one of my old employers where I can choose my investments and only have Roth and Traditional 401k as options from my employer. 

I would suggest looking into a roth IRA rollover, however that in of itself is a taxable event depending on your mix of pre tax and post tax dollars going over to the roth IRA.