As all the boxes practically do the same thing buy the top of the range available within the price you can afford - standard rule of thumb for anything.
Look for built in WiFi - a decent controller, you need mouse ability, keyboard and the standard TV controls and make sure where you buy it from is a well known traceable source.
Once you have the box and set it up, the problems start, this doesn't currently and never will in its current state offer a complete alternative to PPV television. Sport from the major channels is hit and miss and even when its a hit, the picture is pixelates badly.
TV shows are better quality but the box is reliant on third party addon suppliers like Genisis and Phenix - most of the time they have broken links or no streams at all. Not their fault however as they themselves rely on 3rd party servers.
Films - same as TV really
I supposes its a matter of expectations, we are all used pressing a button, seeing a Menu, selecting from that Menu and watching without interruption what we have selected in HD or very good SD.
The Andriod TV box and Kodi provides excellent hardware and software to be able to watch TV over the internet - but the available streams do not always live up to expectations. You will experience frustration and annoyance on endless searches, broken links, poor buffering, disconnects and general mayhem.
I have told many times though, what do you want for free.
After 6 months or so though I have once again managed to set up the T8 - everything but the sport, I conclude that will never compete with the big boys of PPV. I am willing to spend 10 minutes finding and starting the TV show/Film I want s reasonably happy.
If any of the addon developers or Kodi for that matter change anything I don't know who you are but if you are only looking to promote your skill, I can tell you I don't care, I just want stability. What I have however are a very particular set of skills, skills I have acquired over a very long career. Skills that make me a nightmare for people like you. If you leave the software in the stable condition it is now, that'll be the end of it. I will not look for you, I will not pursue you. But if you don't, I will look for you, I will find you, and I will kill you.