What are you after in terms of features?
Headunits at the lower end of the market are perfectly fine for sound quality/basic functionality such as bluetooth, aux, front usb port etc. I usually recommend mechless units at this price point (£50-100).
Midrange headunits would usually have 2 rear USB ports, come with an app that you can download on your phone for playlists etc. They also have the sub-out functionality which will display track information on the interior clock/display.
Top of the range headunits are usually double din units that have a folding/motorised touch screen and support CarPlay and Android Auto.
I've personally always gone for the low end units (bought two different ones brand new from halford for £50) and haven't had an issue with either.