Christian,
Upgrades and mods are highly personal as you have to know what your needs and interests are. Avionics are something I have to think would really help if your Cheyenne is still original. You probably have a good idea already of what you would want to change or improve but I would suggest talking to some avionics shops to see what they say. It is important to know what you plan to keep or get rid of as some avionics will not work with others. For instance, if you keep the original autopilot you want to know if the new avionics will work with it. Of course if you upgrade the autopilot you would want to ensure that anything else you add will work with the new autopilot as well. So that’s where a shop can help talk you through what makes sense.
Now, if you really want to spend money you can look at the Blackhawk engine upgrades! https://www.blackhawk.aero/upgrade/cheyenne-i-ii-iixl-xp135a/
Of course I would only be looking at that if the performance of the Cheyenne II isn’t quite what you want. I’m going to assume it’s already fine for you. But if you want to spend money….
Eric
