What features does a mountain bike need to be converted to an ebike?
The cassette and hub will depend entirely on the conversion kit - is it a front wheel, rear wheel or center drive? If rear wheel, you probably will replace the whole wheel with a choice of swaping the cassette over or buying a new cassette. 

It might not be possible to install a 40 tooth chain ring on some modern mountain bikes. While the 3x bikes of yesteryear regularly ran 44, its rare for a MTB today to run bigger than 38, and 32 or 34 is most common. The frame designers are probably not considering large chain rings as an important feature in the difficult and crowded area around the BB (on Soft tails).  You would need to check the frame to be sure.  

To me, the main consideration is the ability to put an interrupter on the brakes, that cuts the engines when the brakes are applied. This means you will need E-Bike specific hydraulic brakes (Most E-Bike kits have levers for cable brakes with a switch built in). There are after market adapters for hydraulic brakes, but they reply on a stick on magnet and switch.  

Knobbly MTB tires work best on soft conditions, slicks on smooth pavement. The E-Bike aspect changes nothing. (Keep in mind an elite rider can probably output more power than a weekend warrior with a 500W motor). Install the tires that suit the trails you ride.