I think this sounds like a very bad idea. I don't mean to offend you, but your original post displays to me that you don't have the background knowledge you need to cleanly acquire a business, much less run it.
I'm considering buying my in-laws business that they have had for 30+ years. They own a wholesale gas/propane service where they sell/deliver fuel to customers. The business does approx $2.3 million a year in revenue and has 3 employees counting my father in-law. That's pretty much it as far as my knowledge of the company's finances, I know the own the office and some equipment but I'm not sure if there's any debt on their assets.
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My goal is to take over the business but still pay them out of the company's profits so they continue to have income. I will not be putting money down unless that's what it will take to make a deal happen. I also will be working on having a manager/partner run the business day to day over the long term.
This is not how acquiring a business works. If their money depends on future profits, and their interest has not been bought out in a purchase agreement, they are owners. Owners/shareholders get paid from profits. If you are putting no money down, what you are proposing is that they continue to own the business and hire you to run it (or ... hire you to hire somebody else to run it?), instead of running it themselves. If you put some money down, they would be selling some (but not all) of their equity to you, and allowing you to be the controlling or managing owner, while they are the minority or non-managing owner. If you are envisioning that you will pay them for awhile out of profits and eventually phase that out, you are effectively asking them to finance a loan to you to purchase the business.
Simply taking over operations, taking the profits for yourself, and then sharing those profits as you see fit with your in-laws for their retirement income is a recipe for disaster. A relationship disaster, a financial disaster, and a legal disaster. You are essentially telling them that you will take over their financial future and their business (tons of risk to them), while having almost no idea how their business that they've built for decades works, and at NO financial risk to yourself. You could easily run their business into the ground and leave them penniless in their old age. If their total revenue for the year is $2.3M, their profits are far less than that. Your ONE proposed decision here - hiring a full-time manager to run the company, presumably a good one with other options in the marketplace - could easily have a 10% negative impact on profits. That's huge.
IF you buy it, you need to educate yourself extensively. Not just educate yourself on their business, but on business law and accounting more generally. Each side needs their own accountant and lawyer. You should buy 100% of the business. If you need to finance the purchase, do not use your in-laws to finance it for you through a quasi-loan. They should get a guaranteed lump sum that they can parcel that out over their lifetime however they see fit, instead of an amorphous annuity that could vanish any minute.
TL;DR: If they are at all savvy business people, they should run screaming from your offer. There is almost no upside to them, and they would be risking their security in retirement. If you love your family, you should not put them in that position.