Alexa is built-in to the Amazon Firestick, no need for a separate purchase. I was watching ITVx earlier on the Firestick. as long as your TV has an HDMI socket (and isn't a Samsung) all the latest generation Firesticks should work and give you ITVx without any problems.
Samsung have their own version of CEC (where the remote of an HDMI device can also operate the TV nd vice versa) but it seems only Samsung branded or approved HDMI devices will work with the CEC so i nearly sent my second Firestick back as it wouldn't work on my 42" Plasma Samsung TV. However, i also have a Sony (worked fine and that has a Firestick on it too) and a Toshiba (the smart TV with it all built-in but i prefer to use the Firestick as i find it easier than the TV version) - this is the latest Firestick i can find on Amazon and they're doing a deal on price :
https://smile.amazon.co.uk/fire-tv-stick-with-alexa-voice-remote/dp/B08C1RR8JM
Very easy to set up as long as you hve an Amazon account, make sure you use the HDMI extender lead though! A friend couldn't get hers to work when she changed TV, her son had installed it on her previous TV and "lost" the extender lead. There was a screw on the TV case that prevvented the Firestick going all the way into the HDMI socket but using the lead meant it would connect. Also reduces interference from the TV on the stick.
Nearly forgot, here's some info on CEC :
https://en.wikipedia.org/wiki/Consumer_Electronics_Control