I am a multi-property landlord, flipper, vacation rental proprietor among other things as well.

Large units can double for small offices for sole proprietors, particularly when temperature controlled.

(I want to own numerous properties as rentals soon) The LLC is owned by only me, it's set as a Sole Proprietor  right now, but will apply and set as an S-Corp.

The houses you acquire are your inventory, they are not capital assets (tools) of the business.As for operating as a business, whether you are operating as a sole proprietor or through an incorporated entity it makes not difference when it comes to the distinction between inventory and asset.

Most of us Realtors are Independent contractors for tax filing purposes and we file as Sole proprietors or LLC.

And to make it more fun, I just got my real estate brokers license, and am currently set up as a sole proprietor with my brokerage.  

She could sell and you could buy in an LLC that was a disregarded LLC being taxed as a sole proprietor as long as the LLC was not going to file a return and all activity would continue to be reported on your tax return.

For a sole proprietor just starting to build a portfolio, are there any specific questions that I should ask to find the right fit? 

That is kind of why I was wondering if the extra step was worth it since in Arizona I'd be the sole proprietor anyways.

I'm also a real estate agent and investor, and I have read some things about a Solo 401k plan, that I may be able to start and fund because I have self employment income, and am a sole proprietor.