I would suggest having a search around the web - there are a lot of discussions and articles about this topic.
I would suggest to Aeotec Gen5 USB stick for ZWave. There are a couple of options that plug straight into the Pi, but then if you find the Pi is too slow for your needs, you will need to buy another dongle. If you stick with a USB stick, then you can simply migrate to another machine. In reality, for ZWave, all dongles are pretty much the same as they all use the same chipset.
Generally, ZWave devices are much more readily available in the UK than ZigBee, although ZigBee is picking up a little, but ZWave still has a lot more variety and availability.