I highly recommend you test ride a bike with a Shimano Nexus hub before you buy. My Breezer Uptown 8 has one.

Read Sheldon Brown's article - it's true there is a "clunk" when you shift sometimes - I notice it most after shifting into 6th gear: Shimano Nexus and Alfine Eight Speed Internal-Gear Hubs

You can do 20 mph or more on the 8th gear on flats, at least on this Breezer, according to Cyclometer GPS - MapMyRide says I've never done faster than 18 mph, but it also stopped reporting elevation too so who knows how messed up that app is now. Because of my current level of strength/conditioning, it takes me a while to get up there - maybe you can accelerate faster. I'm not sure though you will like the performance as much as you might with a Shimano 105 set or something.