For my part, I will be the initial seed money and overall business operations (finding financing, taxes, general bookkeeping). 

Let's say, it costs you 1.5K to get your structures in place (holding LLC or Series-LLC, with or without land trusts, with or without separate operations LLC) and 0.5K per year (for maintaining the LLC properly, bookkeeping, lawyer and CPA, etc.) for a total of 2K.

I use STESSA as a DIY book keeping solution.

I think you could justify a % return equal to average PM fees/costs if you actually managed the property as well, including all bookkeeping etc. 

When it comes to bookkeeping your properties, I think it depends on how much you can spend your time on managing your property. 

It can be done with comingled funds, it would just cause a lot more bookkeeping and classifying of charges between "business" and personal.

I use a bookkeeper to keep my accounts organized. 

Then you may fall into a bookkeeper category, many learn on the job and have some kind of accounting training.

It does both accounting/bookkeeping and lets me track my portfolio's performance.That said, would I be in the same boat with QuickBooks: I have one invoice that covers both business and personal expenses.

This past tax season my CPA advised me that it was time to get a bookkeeper on board because my Quickbooks abilities were no longer sufficient as my real estate portfolio continues to expand.