Can I use iMessage on my iPad using wifi

Yes. I use iMessage on my iPad Air 2 daily. You will need to make sure the same Apple ID is signed on your iPhone and iPad if you want to be able to send and receive at your phone number as well as send text messages. If you do not have an iPhone, make sure your contacts message the email address associated with your Apple ID. Enjoy your new iPad!

Yes. iMessages will work on wifi though only between Apple devices.

iPads cannot send SMS messages to non Apple devices unless you have an iPhone using the same Apple ID as the iPad.