The STA is a very, very comfortable car (in fact, the seats might be too soft for long distance runs... not enough support...) with great airconditioning, nice leather and lots of toys, but most STAs are now 8 years old.
Check the Idle air control valve, the Mass Airflow Sensor and the alternator and battery... see how well they run... Check that both auxiliary and main fan work.
Make sure there's very little shift-shock when you drive (better if there's none at all)... don't worry if it's slow, that's normal. In fact, it's even slower with the aircon on.
The power-steering pump will usually kick-back at idle or in turns... that's normal... on this over-accessorized engine, load is always an issue, as there are a lot of accessories putting load on the alternator, and the aircon compressor takes a lot of power from the engine (I used to call the "A/C Off" button the "turbo"...

). That's why you need to check the alternator and battery carefully. Listen for sounds from the Aircon and feel for any weirdness... you don't want to deal with compressor problems, as Nissan compressors are usually expensive to replace.
The STA's suspension is very soft, so listen for sounds from the back that could indicate busted bushings. The brakes are pathetic, but they should still stop you. Romp on them once on the test drive (warn the owner if you're going to do this!) to see if the ABS still works.
Check that all the windows and the sunroof work... motors tend to get weak over time. The power antenna may not go all the way down, anymore, but that's a thousand peso trip to Banaue, so it's not a big deal.
The beige interior is a b*tch to care for, check that the leather is in decent shape. Minor interior panel looseness is okay, it's something you can fix at home with a screwdriver and mounting tape, but check that the shifter is still in one piece... STA's have a shifter made of really delicate rubber, and it tends to disintegrate if the driver holds it with dirty hands (from experience). The CD-Changer may or may not work anymore (expected on an old car), but make sure the automatic climate control does, as that's harder to replace.
The STA is like a micro-Cefiro... it's a very comfortable car, with good legs and easy handling. But finding one in pristine mechanical and electrical condition takes a lot of hunting. It's a nice car, if you really want the luxury, but you'll have to put up with the hassle of keeping it in good condition.