Also, your potential employee-purchasers may not want to be saddled with this particular third party--and especially not her bookkeeping business (when they may only have wanted to be involved in a plumbing business). They may have ideas of their own as to how to handle the "inner workings."
Either bring the third party on first, integrate her into your business and then sell the package to the employees . . . or present the option to the employees and let them decide whether they want her to be part of the package.