Re: Specialized Hardrock Sport
Welcome to the forum.
My recommendation for your bike and situation would be to go with a rack-mount kit. I like the Staton product the best. I prefer chain drive over belt drive. I don't think a Chinese in frame motor will fit your bike but Specialized is high quality and has all the stuff I like. Good brakes and front suspension. If you sell this bike you likely won't get full value for it so better to work with what you have. Also the rack mount kits are easier to install and don't require as much savvy to maintain.
The Chinese In Frame kits take a bit more know how and experience to get the most out of. You can always build an In Frame bike later once you get some experience with the rack mount.
Just my 2 cents.