Lump Sum + Percentage on the sales is what my accountant and an SBA advisor recommended when I was in a similar situation. It was slightly different because I was acquiring assets from a closing business (already announced they were closing). From an open business in good shape it will be a slightly different story.
The reality is, for the current owner, best case scenario is they sell the whole business to a new owner. That way they get to markup everything. If he sells to you, you're either not going to want or not going to value a lot of stuff as you probably already have it or recognize it as dead stock. Those same items are often considered vital to the operation by a new owner.
The big thing I gathered from my conversations is you do not know the conversion/switch over ratio when buying another person's client list and files. Some will and some won't. That's why you make it for a percent on the sales. It also incentives the previous owner to push his customers in your direction. If you just hand him $50,000 and say good bye, he could put no effort to help you or screw you over. The basic idea is if you do well, he does well.
With all that said, if I tried to sell my business tomorrow, there is absolutely no way I would take a percentage on the sales over 3-5 years. I'd tell them to pay me some lump some or go away.
