There are many boxes that you can buy out there on Ebay, Amazon, Locally, and many more sites
You need to be careful as to what you are buying. When you are going to buy a box you first need to know what you are going to do with the box. For most, you will use it for TV Shows, Movies, and maybe sports. These boxes can also do much more like connecting your phone to it to either screen share or play your music to it. They also can game whether you download Goolge Play Store games or install emulators to play older games.
The base recommendation for any box is:
The reason for this is that when watching movies that to make sure your movies is not choppy, you want a good Ram and Space. These minimum settings I recommend give you about 250mb of buffering. That is a good number because it allows you to stream that far ahead in your show. A 1gb only allows for about 45mb which is not very helpful.
For Android 6 the reason was this. If your box had Android 4.4.2 or lower, you could not install the newest version of Kodi. This is not good because many of the addons for kodi will eventually not run under older versions. Yes there are boxes out there that have Android 5.1 but by going lower that 6.0, you are limiting the life of your box before you have to buy a new one. Some of these older boxes can be fooled to thinking it has a newer version of Android but most of them can't. Not worth the risk and the newer boxes are usually only a few dollars more.
If you want to play games or screen share, then you should make sure the box has Bluetooth. Also you might want to consider a box that has Dual WiFi (2.4Ghz/5Ghz) just to help you choose less congested WiFi channels.
If you plan on putting your box in a spot farther away from your router than you also might want to look for a box with a good external antenna.