Matt, do you know whether she spends most of the time on the water on mooring or slip, or kept on a trailer? My limited experience tells me kept "on the hard" you'll have less concern for hull issues (e.g. blisters in gelcoat, etc.) which can happen over time in the water, however if not kept covered you have potential of rot from standing water.
I think the "ribs" you read about are also referred to as stringers. My understanding is thare are simply supporting the floorboards, which may be missing anyway. Ours came with a cabin floor, which I took out since there's really not much use for it and it actually decreases the already short cabin (and it was ugly too; somewhere down the road I'll make her a pretty one...)
I'm not certain what type of mast support you have, but if there's an aluminum mast support from the cabin ceiling down, look at the lower end where it meets the cabin sole. Is the wood under the support seem solid? If itsn't it can be replaced, but if it has pushed through the fiberglass at the base that is problematic and a pretty big job to correct.
Also from inside the cabin look throughout and crawl back under the cockpit benches and give a decent visual inspection of any wood (there won't be much) and simply look for cracks, etc. Check out condition of any backing plates where hardware is bedded through the deck or transom for the motor mount, gudgeons, backstay fitting, shrouds, boweye etc.
Look around the CB trunk in the cabin and see if there's any significant cracks and what shape the CB pin is in. To be honest, this is an Achiles Heal for the Mariner and it most likely leaks a bit. If you don't see excessive damage don't be concerened, there lots of creative approaches out there to addressing the leak (a sponge being the simplest...) But if you see large cracks around the trunk that could be a concern.
If it lives on a trailer like mine, make sure you pay some attention to the trailer as well.
Bring a notebook, pen and take lots of pictures. Photograph and write down anything that concerns you or you feel is missing, you likley won't be able to remember everything you see.
Lastly, the best resource out there is the Mariner Class Association. There are some really great, knowledgable and people in this group, with a lot of great information and ideas to share. Check out
www.usmariner.org Without being a member you have limited access, but I think you will find it interesting.
Also, check out Nate Bayreuther's site:
http://www.nathanbayreuther.com/mariner/index.htm
Nate is the current class association president and maintains his own incredible site dedicated to his Mariner
Best of luck and can't wait to hear what you find. I got really lucky and I hope you do too!
Sean