You can get a better answer if you tell us the Roku model you are considering.
This reference will show you the support status of older Roku models. Many models no longer in production still receive updates from Roku:
https://developer.roku.com/docs/specs/hardware.md
What it won't show you is whether some older models being supported by Roku are still being supported by the channels you're interested in. For instance the entire family of Disney owned streaming channels has just dropped support for the Roku 4, although this model still is receiving updates from Roku.
Supporting what makaiguy said, it really depends on the player. But the simple answer is no, not all channels will work on many older devices. If you stick with at least a fifth generation player, every current channel should work, but even that generation is now approaching seven years old and may not be a good choice for the latest channels.
But it's up to the individual channel provider to determine what model devices will work with their channel. Most of them design their channels to work best with the latest hardware, and older devices absolutely see a performance loss with those channels.
Also be aware that those older devices are most likely used. The odds of finding a brand new device that was released 7+ years ago are not that great, and personally I wouldn't buy any device like this used.
