Anything from OEM (Cessna, Beech, Piper....etc) where there is no other source. Anything with a PMA.
You can't just go pick out any old item from the Spruce catalog and slap it in a certified airplane. Take my 170 for example. I can buy a part from Univair for 3 times what I would pay to order it from Spruce, but he Univair part comes with paperwork that the Spruce one does not.
Consider how many OEM unique parts that are really in the typical airframe, very few. Most all airframe parts that are unique to the manufacturer are folded up or flat sheet metal which can be reproduced at normal industrial prices. There are a few castings and forgings, but with few exceptions, there's enough of these parts in the junkyards that you will never need to go to the OEM to get them.
To me it's pretty much the same either way, certified or experimental. Why do I own a certified plane? There isn't a high performance experimental twin I can afford to purchase/build by a factor of over 10 fold what I have in my 310 even given the certified avionics cost when I'm all done with an AP and full glass.
For a different mission I would chose an Exp as I have before if it was best suited for the overall mission.
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