The Canadian Border Services Agency enforces a 2002 law that prohibits entry into Canada if you have a criminal conviction. Your expungement is not really an expungement. Under 1203.4, the NCIC records (which are now being shared with Canadian authorities), will still show the arrest and will show the dispositionof "1203.4 dismissed." They will probably know how to interpret this just as federal and state law enforcement does in the United States.
You can seek Approval of Rehabilitation but that requires that you show a stable lifestyle and that you are actually "rehabilitated," that is, not likely to commit another offense. This requires that you wait at least five years from the end of your probation so you are much too early.
I believe that you can apply for a temporary permit, even with a criminal conviction, but that you have to show that there is some sort of emergency or urgent reason why you need to enter. I have also heard that there are some less formal ways to be allowed to travel in Canada using a "adjournment" procedure where the person is not eligible but the CBSA decides to let them in, set deportation procedures and then have a temporary adjounrment while the person is released and can travel about for a specified period of time. But this reqlly requires the assistance of a Canadian lawyer.
To answer any of your questions and to get actual legal advice, you should talk to an immigration lawyer. You can also call the Canadian Border Services Agency (CBSA) for more information.
This is not legal advice. In order to get legal advice, you need to retain a lawyer and establish an attorney client relationship. So, talk to your lawyer! [I am licensed to practice law in the state of California and am admitted to the federal courts in California, Washington D.C., the Ninth Circuit, the U.S. Supreme Court and a number of other federal Circuit and District courts. For legal advice, you need to retain a lawyer in the jurisdiction in which the matter is pending.]
There is a cruel lie that some attorneys continue to preach to felons. I am not referring to Mr. Sanger. People are sometimes desperate to clear their records and legal low lives tell them that a 1203.4 will give them a fresh start. Some even say it will seal your record. Nothing can be further from the truth. Your arrest and conviction remain on your rap sheet with a notation that the conviction was dismissed. This has the value of a bucket of warm spit. Sorry but you should change your travel plans.
Your expungment will mean nothing to the Canadian Border Patrol. The expungment will still show that you were arrested and convicted. It will likewise show that it was subsequently dismissed. Contact an immigration attorney in the US or Canada if you want further information.
Legal disclaimer: You can learn more about me by clicking on my Avvo profile. Disclaimer: Please note that this answer does not constitute legal advice. It is merely intended to provide general information to aid the poster in finding answers to the problem posed. This answer does not create an attorney-client relationship. In most cases, it is best to contact an attorney directly to find answers to your problems.