None of the Solo 401k plans offered by big companies like Vanguard, Fidelity or TDAmeritrade allow you to invest in real estate or any other unconventional stuff other than stocks, mutual funds and etfs. Neither do they allow you to do in-plan roth conversions.
Unless you are going to invest a few hundred thousand dollars and prepare to do a whole bunch of paperwork and pay a whole bunch of money to the plan administrators, I would stay away from this idea.
On the other hand, I like Solo 401k because it lets you defer up to 55k or so of income, thereby reducing your tax liability now, assuming you are in a high tax bracket.