I don't know about private businesses and this is NOT speaking from experience so take it for what it's worth but my thoughts are:
- You should be able to buy it at much lower valuation than public markets (S&P500 long-term P/E is around 15 and I wouldn't pay anything more than 3-5x PE for illiquid, small, private businesses; I'm just using P/E as a reference but some measure with EV is probably a better measure) but stability of revenue/earnings and how the business is financed (especially if it relies a lot on bank loans that may come due) is probably more of a concern
- You may have already done it but you need to spend more time on qualitative risk elements that don't show up on financials. For instance, is the current owner also the chef (what happens if the chef leaves; will the food be the same)? Do you need personal relationships to survive (is there a deal the current owner cut with suppliers/landlords/bankers/etc that you can't get if you talked to those parties on your own)?
- You mentioned 25 years of history but is this more for a common food that will last for 10 years, or is it something that might lose popularity
- I would also make sure to factor in costs that aren't recorded in financial statements that are real and related to the amount of time the owner or their family puts in. Many small businesses look ok but are terrible when you factor in the fact that the owner often works long hours without getting any (or minimal) salary (many local grocery stores, dollar stores, etc are actually poor businesses and barely earn their cost of capital if you account for the almost 7-day workweek that owners or their relatives put in).
Good luck with your idea. Hopefully others can provide more input.