...and a swirl inlet dip tube - or you can modify the factory one. Keeps sediment from building up in case you're not rigorous about flushing the tank occasionally. Your choice about condensing burner or not, but I like my water heater to work even when the power is off... finally, you might consider a 50 gallon instead of a 40 gallon. AO Smith makes good heaters (no affiliation) and so do others. Warranty duration is one indicator of quality, but not the only one.

New ones are supposed to have a flame arrestor screen around the burner chamber to prevent ignition of flammable vapors and gasses creeping along the floor, but you might be able to get a better deal on an "old" model without the screen if you bargain with the supplier.