What i really need to know is will I need a visa and how long will it last.
As a holder of a British Passport, you will not require a VISA to visit Canada for the initial 6months. You will however need to apply for VISA extensions should you want to stay beyond the initial 6months period.
What will I have to do to stay in Canada if we marry
Should you marry a Canadian citizen or PR of Canada, you can use marriage as the reason to apply for VISA extension. Unless you are inadmissible due to health and criminality, there is little chance to be refused for VISA extension under marriage.
What are the major obstacles
That really depend on your specific case. Presumably you would want to become a PR or Citizen of Canada so you no longer need to apply for VISA extensions. If your future wife is eligible, she can sponsor you through the family class. You may also apply through the economic class should you qualify for it.