I’m currently using QBO but in discussions with a colleague it was recommended that I get QBD for the enhanced functionality.

The primary reason that LOCs secured by investment properties do not exist is that investment properties are exchanged so frequently.There are "LOCs" available for fix/flip loans, but in effect they are function more like a pre-approval for a fix/flip loan (rates/LTVs/Fees usually are pretty decent on these).

I've been a user of the QuickBooks Pro desktop version since 2002 and use the "class" function.

Would the money market account function as my business checking or would I still need both?

It functions mostly to give us the business structure we need.

Playing word games doesn't change that fact; and trying to have-wave it away as a function of an engineer's focus on pragmatism is even more absurd. 

I am located in Greenville after a long brokerage and investing career in Silicon Valley - also spent ten years in Forest Hills Queens.Greenville is more of a capital growth market an appreciation market rather than a cash flow market.Clearly, investors want both cash flow and appreciation, but I have found the relative weighting given to either is a function of where an individual is at in his or her career, income level, and particular needs.I believe (my opinion and I understand others think differently) is that early on and into peak income years - the proper focus is on appreciation - growing that nest egg   long term - one has sufficient income and does not need income from RE to live.

I have a house where I replaced the siding, windows and doors, and wrapped all the trim with metal, but did not replace an older roof that is still functioning fine.

It is functionally an 18 unit building, and I'll say there's a ~100% is appraises as such. 

Adapt and overcome, keep adapting and keep overcoming. agreed folks adapt.. or move on to something else.. it could cause social and functional obsolescence  and you would end up Like Detroit areas were they bull dozed whole blocks of homes since they were no longer feasible as housing units..