I think the only thing that KC actually planned was leaving Hopespring Drive, to get away from Cindy. And I think she discovered it would be too awkward to take Caylee with her. I do not think she duct taped Caylee's mouth to attempt to control any purging - I think she did it, deliberately, to kill Caylee. She would not want to touch her baby if she were decomposing.
She had her little bit of money with which to esacape - probably to California to be with Mark Hawkins - far far away from Cindy and her stifling control. She just needed one more day, to make her final escape.
She said in her letters that she was buying things for her trip - that did not happen anyplace but in her mind - I do not think she had enough money to buy stuff and to leave too.
I just do not know why she had not already left, unless it was that she was afraid and nervous because Cindy, even though not physically present, still had inhibiting control over KC. Just look - KC was legally an adult. Why did she even go with Cindy when Amy brought Cindy to TonE's apartment?
I think she intended to retrieve the Sunfire from Amscot - she just was not fast enough, probably because she thought the stench needed time to subside after she had removed Caylee to Suburban Drive. She did go back to get it, but it had been towed by then.
And the 'purse' left in the car? It was not a purse, but more like a tote bag. There was really nothing in it that she would need. There was no thought behind leaving it the car - she just had no use for it, just like the boots left in the car. She wasn't wearing the boots, not using the bag, just no motivation to carry them inside the house, so she just left them in the car.
I do not think she planned anything other than to get away from Cindy.
All, of course, just my opinion.