The best time to request a background check is after you’ve done an initial screening but before committing to a lease.

My initial plan had been to do something like Rent-to-Retirement or REI Nation- assuming that I'd be leaving money on the table to pay for their expertise, but hoping that I would learn through the process and develop the confidence to strike out on my own later. 

It's not clear in your post how heavy of a lift these entitlements are but assuming value is being created before a shovel enters the ground the initial LP investors will want to be compensated and share in the upside that's been created.

I'll be looking for long term rentals that don't need any major repairs initially. 

Quote from @Mark Sullivan: Quote from @Tim Delaney: One missing piece of info here is how much cash you have available to invest at the moment.My initial thought was sell LLC B and hopefully you have a few hundred thousand available on top of that to buy the new building.

In terms of creative ideas, you could look into leveraging that construction experience and partnering on a deal to ease the initial capital need.

I'd be seeking someone to fund the down payment of the deal and I would be responsible for property management/bringing in funds for initial renovations for a profit/split.

My initial reaction is that it costs $150,000 to add an extra door in a premium location, but I am interested to hear thoughts. 

With 400K my initial thought is doing a BRRR/Flip that is close enough for you to be hands on.

It is easy to initiate a trustee-to-trustee transfer between firms.