When my business partner and I were still flipping we bought a couple through Auction.com and we always used a private money lender to fund those and our other flips. 

That is tight leverage for an investment 2nd position loan.A private investor might be willing to do it, it's not going to be cheap at all & will be short-term money & will not be a line of credit to take out at your liking.What is your rate on the 1st?

(One went with another manager eventually, and the property is listed for $400/month less than we listed it, and the other finally got rented to a solo professional.)

Most professionals will recommend a separate LLC for every $1 million in assets but I don't think that's necessary.

Or just use private money. 

Find out who has good Google Reviews (and responds to them), find out who answers their phone, find out who has a polished and professional website, etc. 

However, once you decide to hire a true tax professional (and not one from AL), you will discover that your $1,500 tax preparation price without a partnership suddenly turned into a $3,500 price with the partnership.

I’ve realized I need more passive options, and have moved to a syndication and private lending.

I found this property through my network of local real estate professionals.

Definitely get with a qualified tax professional on this.