Let me help you answering those questions so you can use your Slingbox to view your Portuguese channels from Canada.
1) The slingbox-pro HD supports PAL, or would I have to purchase it from another country?
1b) Is it possible to order the PAL version to Canada if needed?
You will need to install the Slingbox in Portugal. The TV signal there is PAL B/G, for this reason you will need to purchase the Slingbox in United Kingdom, so it will support the same TV signal. The Slingboxes sold in Canada or United States uses NTSC, so those will not work.
2) If the power goes out does the slingbox automatically restart/ or can be turned on once the power is back?
The Slingbox needs to be ON, in order for you to view it. Sometimes, after a power outage the configuration might be lost, however you should be able to connect to it again without any problems.
3) How well would it work for such a long distance? Network speeds shouldn't matter.
You will need sure that your network in Portugal meets the system requirements, therefor you could watch it from a remote location without having any audio or video issues.
Check this link for additional information on the requirements:
4) Anything I should know before attempting this? I just recently heard of slingbox and don't know to much about it.
After you set up the Slingbox in Portugal you should be able to connect without any problems from another remote location.
I hope it works. Regards.