To answer your questions:
1. I'd really need to know more about the type of cruising you expect to do. Weekending, long term, full time? What kind of wind conditions do you expect to be sailing in? Are you going solo or will you have others aboard?
2. Replacing headliner is a dirty job but not hugely time consuming or expensive. The hardest part will be getting the old one without creating a huge mess. There are numerous threads on this site about headliner replacement. A bad headliner wouldn't necessarily deter me from buying an otherwise sound boat - though it might make me more cautious if it was severely water damaged/stained.
3. She seems happiest in winds of about 8-18 knots. Pros - Points quite well for her design. Doesn't have a lot of excessive weather helm. Doesn't have excessive heel - I've seen other boats bury their rails on those 8-18 days where I've been hard pressed to do so. Base PHRF is 180 on the Chesapeake but I believe she's faster and the 180's generous. She'd probably keep up with boats based in the 170's. Cons - Not a great light air performer, but she will keep moving. Foretriangle is a bit large compared to mainsail size so even with a 100 jib and reefed main you still have a lot of headsail out.
Beyond that, I'd be a little concerned about the staining on the cabin sole you mentioned on the boat you're looking at. Usually a stained cabin sole is a sure sign of excessive water having entered the cabin.
Beneteau Oceanis 400
formerly Lippincott 30