I agree with smurph, but in some ways, since this is your first sailboat, you won't really know what your wants/needs/desires are/will become.
Don't forget to test the engine. See if it will start in 2 or 3 pulls (even if it's an electric start). Have you worked a little outboard before? Make sure the engine puts out a steady stream of cooling water out the telltale. Keep the engine down at idle, and put it in gear, then slowly increase the throttle to make sure it doesn't stall. Can you see the prop spinning (or the water being pushed by the prop)?
Also, the soft floor is throughout the entire boat. That includes the cabin top (the "deck") and the cockpit floor, plus the cabin. Make sure you don't wear squishy shoes, or take your shoes off as you're walking around on the boat. One of the boats I was aboard had BIG soft spots. I had never experienced one before that, but once I hit one, I understood what they were. You'll know as well.
You won't always be able to tell if the chainplates are rotten. So look for signs of water leaks, like wet/stained wood (many chainplates attach to wooden bulkheads) or soft wood around the chainplates. Take pictures of anything you think is questionable, and come back and ask.
Since the owner won't be there, I think you're in luck. This is a great opportunity to start familiarizing yourself with sailboats, and you can take as long as you want to do the inspection. In one of my earlier posts, I linked to the Catalina 25 self-assessment list and the thread from here about self-assessing a boat. Read those both 3-4 times. Take the Catalina list with you, and walk through it step-by-step, even if you're ready to run screaming from the boat the moment you are aboard. You'll be glad you took the time on this boat, because on the next boat the owner may be hovering over you, trying to sell you and to steer you away from problem spots (or at least distract you from them).
Be sure to bring a flashlight or two (I donated one of mine to Poseidon's toolchest when I was inspecting a boat), a notebook, and a digital camera.