The memory card class depends on the camera, our HD camera needs class 6.
Make it part of the checks you do when you look at the camera - what speed card does it need and can I buy any ?
No point getting a new camera if they can't sell you memory cards to work with it

I would avoid hard disk cameras like the plague, they aren't robust and it's easy to damage the hardware. If it's totally solid state and has a few gigabytes of SSD memory instead of a hard disk then that may work well for you, but those cameras are much more expensive than memoryless ones where you just slip in a SD card.

Tess now has her own website: http://smartwombat.myphotos.cc/gallery/Tess