The profit sharing contribution should work, assuming the plan document language supports this sort of allocation.
It is common to credit prior service with a selling entity for employee eligibility and vesting in the buyer's plan, so nothing unusual there. Assuming they are buying the assets from the seller's corporation, the seller won't have ownership in the buyer and the selling entity is not contracted to provide management services then the seller should be an NHCE.
Cash balance won't work as they have to meet 401(a)(26) participation requirement.
If the current safe harbor matching 401(k) would be top heavy but for the SH exemption, adding a profit sharing contribution to the plan would blow this exemption, so TH minimums could then be triggered due to deferrals and match received by key employees in that plan. You could avoid this with a separate PS plan that covers only the seller. Since he is an NHCE and non-key, and the 401(k) does not rely on the PS plan to pass coverage or non-discrimination the two plans would not be a RAG under 416 and the SH plan could maintain its TH exemption.