The key is the type - I agree. In general music MUST BE suitable; focus is in your hands - what is your target - what you'd like to show; if music is only background - the foreground is picture; what if the music is more expressive - pictures becoming background.
I've got many types of cars from many countries and finally I've decided to match the music from these countries to the cars are originated. And the type of the cars matched with the type of music - for example if this is race car - music tells it - plus some sound effects mixed with music. And the tempo as you said must be right.
50 minutes show with several 'chapters' seems great.
Instrumental is more suitable generally but in some 'situations' vocal makes the feel. With suitable words of course.
But I agree with you - in balance: background/foreground.
Instrumental is more 'imagineable'
for me.
My second show (about 20 minutes) is in trance style. I've filtered pics with a bit 'dreamy' filters. Only instrumental is ok for me in this style. So all depends on the style you choose, I think.
R