Corrosion is always at the top of the list. Making sure the old black duct (CEET) is removed through out the aircraft and replaced with heavy-duty smooth-lined SCEET ducting (called 'SCEAT').
Landing gear donuts, confirm via logbook when they were last replaced and look for notes on jobolt replacement while you're at it. Make sure the pre-buy gets the plane in the air to check the gear movement and knee pin sleeves.
Also check the air intake plenums by the windshield, directly under the vent between the windscreen and top cowl. if they are cracked they may have caused corrosion.
My pre-buy went very well and I ended up flying my plane home from texas to Delaware to have an annual done by the guy I have been flying behind since I started, it was just a comfort factor for me.
My annuals vary but never to extreme. you can find the write ups on my blog listed every november. I try and do upgrades every year and stay ahead of items. Take this past annual, I had some play in the knee pins on one main and the nose gear, and did all three. Upgraded baggage door seals and things like that.
Our Sundowner is a good solid plane that my Bride and I enjoy traveling in. Hope you find one just as nice!