Buying phone number and customer list

We are running a commercial mechanical business. We have been talking with the owner of a smaller shop in town who is planning to retire this summer. It is not a business worth buying. However, I see value in hiring his 2-3 technicians and buying their phone number and their customer list, mostly for the service work that they have been doing. They dont have service cotracts in place. Can you share some insigths on how such a transaction can be structured? How would you price something like this? I was thinking that we would pay x% of the sales to anyone on his customer list (assuming they are not already our customers) for a period of time? What would a fair price be?