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@Anglis wrote:
What I am looking for is what is the best way to treat the $2000 monthly income paid to father on his personal tax return. I don’t want to complicate this.
The son and partner do the business now as a new business, default to partnership. So the monies being paid to related party should be their capital investment basis, am I wrong here?

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Are they paying $2000 monthly until somebody dies?  You need to determine what the actual selling price is to calculate the gain.  On the flip side, the payments aren't a capital investment.  The debit side of the transaction is an asset (client list) and the credit side is a note payable.  The monthly payment is purely a debt payment consisting of principal and interest.