Check how bad the scratch on the underside of the front spoiler is (usually drivers side). If there isn't one, then either you've found a very carefully driven car or it has been replaced.
Pop the engine lid and look at the intake and the top of the engine. If the metal looks sort of powdery (hard to describe, but obvious when you see it), then my experience is this indicates a car that hasn't been garaged. I think it's caused by rain water entering through the vents in the engine lid and sitting in there.
Make sure you get a car with AC. The targa roof adds a lot of glass area - it gets hot inside. Ensure the AC is cold. After you try out a few you'll know how it should perform.
Check that the two sun shade panels, which fit under the glass targa roof, are present. I've been told these are very expensive to buy from Toyota (warning - 2nd hand information). There should also be two bags for storing the roof when it's off.
And get some insurance quotes. You may get an unpleasant surprise.