If you just want to try it, then rent them a few times to see how much you really like it. If you want to actually make it a method of fishing, then spend the way you would on your other serious pursuits.
There are free demo days all across the province this summer.
One thing for certain is that you will have no problem getting rid of a good fishing kayak, should you decide later on that it isn't for you.... that's why you can't find a good used one.
12' or shorter for small rivers, small lakes and ponds. 13' and longer for larger rivers, lakes and the great lakes.
good luck!