Don't cheap out on the duct size, moving up to 5" or 6" will let you put in a much quieter model and still move the same amount of air.
The corrugated plastic conduit gets NASTY after a few years of moving moist air. Mold city. The straight metal tubing seems to have less problem with that.
I've replaced a bunch of Broan/Nutones over the years, but never any of the Panasonics. I highly suggest making sure you can get a drop-in replacement kit for whatever model you choose, you don't want to have to rip out part of the ceiling in the future to replace the unit.