I can't give advise on gear ratios, wheel sizes, crank lengths, etc.... I do not remember what I have on my own rigs. But from our experience:
-Chain rings, chains and cassettes are parts that you will find yourself replacing often. I doubt that you will be only riding 4000 km/year once you get your new rig.
-We have more than one cassette that we switch for especial rides.
-You can use 10 sp chain on a 9-speed cassette. Don't know if vice versa??
-We can spin OK at a max of 120 RPM so we can develop 41 MPH... very close to the top speed that my wife feels comfortable at.
-We rather limit our top speed downhill than having a hard time going uphill. Knees on one piece at the end of the climb, and a happy stoker, are much more valuable than a few seconds difference in a long down hill.
- It never bother us before, but as we age, small jumps between gears are greatly appreciated.