Congratulations on connecting with someone who wants to help you buy the business.
As far as how much ownership the investor should get in return for his investment, that would depend upon several factors. You say he doesn't want payments but you didn't say how he expects to realize the benefit (return) on his investment. That could be a key factor in deciding how to structure your arrangement.
As you three discuss the details of your agreement - do remember that ownership (equity) is only one part of the equation. You should also discuss:
1. Control - how "strategy" decisions will be made and how the operational decisions will be made.
2. Results - how will the results be distributed
3. Compensation - If you are working at the business, how will you be compensated for your efforts
Hope this helps. I realize this is a general answer, so if you want to discuss the specifics of your situation- feel free to drop me a PM or contact me directly.
Business Growth Masters, LLC -
Capital Catalysts for Entrepreneurs
Home of the Scalable Business Plan
and QuikStart Capital Programs