You can get an S5 on any tab you wish, so in this case yes you can get it on the cheapest post paid plan Koodo offers, just add $5 for a tab charge to your monthly bill and you're set. Keep in mind that the cost of the phone is $425 with a tab medium, but it is the cheapest option over the course of 24 months.
You definitely can. What I don't understand is why people want high end, expensive phones and then only use them for cheap talk and text plans with WiFi only. At that point you're better off with a less expensive model like the Nexus 5, which performs roughly at the same level as the S5 but at less than half the price, albeit a not as good camera.