IMO a foundation issue is not a deal killer. Here is what I have done. Get a professional foundation inspection. I use a foundation repair company and they did the inspection for free. With the foundation estimate in hand you have huge negotiating leverage. The foundation is now a known issue and the seller is obligated to share this information with all future buyers. He is stuck dealing with the issue. You can either ask him to make repairs or give you credit on the sales price to make repairs. I did this recently on a 4-plex and was able to get $27k price reduction. After closing I was able to make the repairs for $12k.