71-Nacelle/Cowl
73-Engine Fuel & Controls
74-Ignition
78-Engine Exhaust
79-Engine Oil
80-Starting
The choice of power plant options seems fairly straight forward. An IO-320 160 hp Lycoming seems to be the engine of choice for most recent -3 builders. An injected engine would provide the power, aerobatic performance and efficiency that is desired.
In considering a fixed vs a constant speed type propeller, I am not convinced that the added performance of the constant-speed prop is worth the weight, cost or complexity involved in that type of installation. At present, I am leaning toward the simplicity and weight saving advantages of a fixed-pitch prop.
n46at 
Keep it simple like Van did in N17RV and it will work fine for you the more systems the more problems to contend with. Take a look at an Aeronca Champ and its systems, basic and worked great. Even knew a fellow and flew with him that flew one IFR back in the 60s and worked fine for him at that time. Also was a senior Pan Am pilot out of Seattle. Times have changed for that type of flying but just saying.