Well, it all depends.
The first thing I would do is consult the dealer for your question at hand. Visit the parts desk. Get an understanding if this a doable project. Next, get a price on all of the required parts from them. You will need this to then shop around for aftermarket kits that upgrade to 134a.
You could yank out the A/C system and drive with the windows down during the Canadian summers. Sure, a few days will push into some high temps, but it's not all summer long like NC, USA. Does your van have powered windows? Even aftermarket power window kits are available too.
Those are some thoughts. A few years ago, I took out the A/C R12 system out of my previous 1985 Dodge Caravan. It had powered front windows and the pop-out rear windows to keep cool.