As mentioned, leaks from rain water are common around the chain plates and windows and maybe the deck. The first are easy to fix. I'd be sure to have them put the boat in the water and out if it isn't all ready in the water. If it is in the water I'd like to see them retrieve it to the trailer once so that you could give the trailer a good look over and make sure there are no surprises there. In the water you would get a better idea of the sails, outboard and the rigging.
Our biggest single outlay was for a new outboard that had electric start so that both of us could easily start it. Next was for new sails, but we did sail (or tried to while learning) on older sails. If the running rigging is old that can be an expense also, more that one would think.
The trailer should also be inspected for extreme rust. Minor rust can be dealt with and other items on the trailer, such as springs, shackles, hubs and such can be replace for not a lot of money but hopefully the trailer is still functional.
I wouldn't necessarily steer away from a boat with mods if they have been done in a manner that will benefit your use of the boat and are well done. I've done a lot of mods and my boat would not be the best for someone who only wants to day sail but if someone wanted to do longer trips it would be a different story.
Decide how you are going to use the boat and look for mods that will make that more enjoyable. How do you see yourself using the boat?
I love my S and have no regrets buying it and for my/our needs it has been the perfect boat,
Sumner
=======================================================
1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...
Mac Trips to Utah, Idaho, Wyoming, Canada, Florida, Bahamas