The boat you are considering is a bit of a rare bird. Not too many Oday 25's were made with fin keels; the tall rig is somewhat less rare, but is still definitely non-standard. Basically, the model you are considering is equipped with "options" that make it less trailerable, one of the key selling points of the boat. Because these features are not common for the boat, it may be difficult to find others who can give you advice about what to specifically look for in the keel and rig.
For the rest of the boat, here are some common things to check:
-Condition of the bulkhead, especially where the chainplates attach. Water intrusion thru the deck via the chainplates will get to the bulkhead. If not fixed, the water will rot the bulkhead, making the chainplates unstable. Fortunately, the chainplate attachments are out in the open and easy to inspect. If the bulkhead is punky around the chainplates, the bulkhead must be replaced.
-Pintles and gudgeons: Oday was notorious for using undersized hardware to attach the rudder to the transom. Check the hardware, rudder and transom for signs of stress. If the hardware hasn't been replaced already, it may need to be.
-Ports: if the ports leak, they are a real pain to replace.
The only other "usual suspect" when looking at an Oday trailer sailor of this vintage that I can think of is the the centerboard. You don't have that issue to deal with here. I don't know how Oday attached the fin keel on these boats, but I wouldn't be surprised if Oday covered them up with epoxy. If so, you will have to take it on faith that they are in good condition. Check around the keel/hull joint for any stress cracks or compression, indicative of keel movement or hard groundings.
Will you be having the boat surveyed too? If you are spending over $4 or $5 thousand for the boat, its worth it.