Google Voice, Skype and anything else is better than paying the postpaid wireless rates for international, so that isn't a downfall.
Yes, you cannot take the phone outside of the USA and expect it to work, Straight Talk doesn't provide roaming outside of USA on these plans. Once again, postpaid wireless rates inside Canada can be expensive, just get a local sim in Canada and use that. I'm sure there are cheap options.
MMS takes few seconds to setup. The only time it's a problem is with iPhone 4GS where changing the MMS settings is blocked and in that case the phone must be jailbroken.
You will be getting MMS without a problem on your iPhone 3GS.
Straight Talk will be a great move for you and you will save up tons.
Be sure to look up on how to get Straight Talk for a flat $45 fee (no taxes) - it's discussed on a different thread.