Get a model-specific unit, highest spec you budget allows, preferably octa-core, 4GB RAM, 64GB NAND (which Chinese insist on calling ROM, even though it has nothing to do with Read-Only Memory, like CD or DVD), and latest Android OS (less important than the spec).
Just make sure you are buying from a reputable seller with UK warranty and return options, etc - these units can be hit and miss reliability-wise.
For what it worth, I fitted mine over three years ago and it has been flawless.