Avionics...
It all depends. There is no standardization of the fleet. I know that most BE-58's have GPS, quite a few have WX radar, almost all have HSI's, some autopilots, and other than that, it all depends on how the previous owners equipped them. You will at the minimum have DME in the Barons.
The PA-31's are a more standard cockpit, but still not guaranteed to have any particular equipment. By standard, I mean the engine guages are all in the same spot, etc. I think most have autopilots, some have radar, GPS, etc.
Caravans are a little better equipped, except some of our out-based aircraft. I have heard some are basic models with very little in the "bells and whistles" department.
As for the 310's, I do not fly them, but I have been told very few have WX radar, and it varies on their equipment.
The nav systems in the A/C I fly have varied from different GPS manufacturers, RNAV systems, and LORAN.