Hi welcome to our forum.
The HDRxxxxS boxes (1000S. 1010s and 1100S) have two tuners and an internal hard drive for recording. They can all record two programmes at once and have the capability to view a 3rd programme while recording two depending on what you are recording. For full capability you need two satellite outlets (They can be used with some limitations with a single satellite outlet as they can use both tuners on a single cable). The HDR stands for High Definition Recorder. Apart from some hardware differences they are very similar.
The HB1000S is a much cheaper single tuner only model with no internal hard disk. It can record and pause live programmes but only if you connect an external usb drive. Being a single tuner means you can only record one programme at a time and watch a limited number of second channels). It only has and needs a single coax connection.
All of the above have the same On Demand capability.
If you are happy with recording only one channel (and using the TV for viewing while recording), the HB1000S is by far the cheapest option.
You can buy a HB1000S graded model from Humax direct for £49.00 pounds.
TV Boxes
Add a usb drive for around the same price (depending on the size) will cost about £100.0. If you need to record two at once and view a 3rd a HDR series box is what you need.