Buy into business

I have an opportunity to "buy into" and expand an existing business... I will use the name, process and all the contractors to complete construction jobs. The problem is that I can't figure out the best way to structure the offer. I will be given qualified leads/referrals from past customers and any assistance from the owner while I expand the company to a new area of town. Do I offer a percentage of the profit of each job and if so, how long or up to what value? The new area will have no prior sales due to the current company not doing work in that area (yes, they have been turning down the referrals that are out of their area) so it's hard to come up with a value of the new location.