Inspect the trailer as well, especially the wheel bearings, and since you're there already, repack or replace them. Tires good? Look out for rust on the trailer that would cause you to lose the boat.
If this little boat checks out, I would jump all over it. Just be prepared to fork out cash on a regular basis.
Very good point. The leaf springs on the trailer can also rust out and need replacing and the tires do not last forever.
Things to check on the boat itself:
- chain plates - if there is any rust bleeding out from these it could indicate a problem. This is fixable but not a quick and easy job.
- center board cable and pivot pin (if the Mac 21 has this)
- pintles and gudgeons for rudder
- standing rigging wires - should not have 'meat hooks' or broken strands
- soft spots on deck
- motor mount - it will hold a motor if it can support your weight
- running rigging (lines) may be old but can be replaced (hold on tight to your wallet)
- general overall condition or obvious flaws - will indicate how well the PO took care of the boat
One good thing is that MacGregor is still in business. They don't make the 21 footer anymore but may offer some support for it. There were a lot of those made.
Above all, have fun you crazy kid!