For the most part, a first time DUI offense is charged as a misdemeanor, not a felony. There are some exceptions to this rule. For example, if a drunk driving accident caused someone to be injured, the charge against the driver could be raised to a felony.
And yes, if you have a DUI Canada says no go about coming into their country.
It can be either. A DUI is a gross misdemeanor unless you have certain priors. If there are injuries, it could be charged as vehicular assault, which is a felony. I can answer any questions you have. Feel free to PM me.