Can you have iMessage without cellular

Yes, but you will NOT be able to send/recieve SMS messages without the need to tether your iPad to an iPhone to send and receive these SMS messages on an iPhone, through to your iPad.

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

Yes, it uses wi-fi and your Apple ID. If you have an iPhone, you can use Text Message forwarding and also use your phone number.