The balance will pay for 3 things on the seller's side: the listing brokerage, marketing and - finally, the agent's profit (and taxes). 

However, how many plots could I fit on 2ac of land without being obnoxiously close but also maximizing the profit potential? 

Can you develop an online sellers calculator that would tell them how much profit they get from their apartment sale?

We will roll profits into apartments 4 income How did you find this deal and how did you negotiate it?

Do you think it would be profitable or just a headache?

And we got lucky again because the house never burned down and we sold it for a nice profit.

your credit, and history, experience plays in to the rates, then 3. does the property cover all expenses with profit. 

(sellers request also, but my preference also bc nobody knew each other, the mark up/fee/my profit.

Take the profit and go buy a bigger office building (that could handle 15-20) units, or an apartment building if you don't mind residential tenants.BTW, she does not want 10% equity, otherwise she would put 20% down and buy the building herself.

To really maximize your STR profits, you need solid year-round demand.