Epru,
welcome to the forums. Looks like a nice boat you are looking at. I used to have a 1984 260, and it had the stress/gel cracks. I've seen a lot of Stingers, both pre 1986 and post 1986, and all of them have had stress cracks. I believe it's just the age of the boats themselves.
I think the most important part in buying your boat, however, is to make sure everything is sound. Specifically, you should make sure there is no rot in the transom or stringers of the boat. Also, you will want to check for any moisture in the deck. Check the engines - check the bilge to see if it is clean, and I believe you should have a compression check done on the engines. Check the outdrives as well - check for nicks, scratches, bent parts that may indicate they hit something. Check the bottom of the hull for any gouges, scratches, or blisters. You should have a surveyor take a look at the boat and let him tell you what he thinks. It's going to cost you for a surveyor, but a good one may save you a lot of money.
I've never seen anyone section off a toilet in the cabin - and I've seen a lot of modifications. I'd think you'd find that there is not a lot of room as it is in the cabin. Maybe, as a suggestion, you could consider some sort of curtain hanging from the ceiling surrounding the toilet to add some privacy?
Good luck with the boat! Would love to get more information on it if you have it.