For those who have completed aircraft and to those building, what considerations have you made towards engine and propeller choice and why?

I know a lot will depend on mission profile, an element which I would also be interested to hear.

I'm based in the UK, where on the worst day we would never see density altitudes above 5000 ft, even in the highest terrain. The jury is out as to whether I would every use the full potential of the 915is and 3 blade MT constant speed propeller? The ability to cruise at 10,000 ft with reduced fuel burn and maximum efficiency does have its merits but defeats the LOW and SLOW profile.
The cost is eye watering, another consideration. You could buy a lot of fuel for the difference between the 912is and the 915is.
I do like the idea of a constant speed propeller in that it does give access to the full potential of the engine, albeit with added complexity.But with which engine?

I'm sure there are aspects that I've not considered. ANY inputs would be gratefully received whilst I chew over this most difficult of choices.

Many thanks.
Ratc