Yeah, don't go cheap!
Make it a 3.5" mains-powered case, (ie not a 2.5" self-powered) I use a Welland eSATA/USB2 case I had spare.
I have a green-power Western Digital 7200rpm disk that works well.

The things you need to have are:
1. When you plug it in a (Windows) PC, make sure it shows up as a "Hard Disk Drive" not a "Device with Removable Storage" - if so, you will (may?) not be able to use it for timeshift PVR recording.
2. It will get formatted to FAT by the PVR unit, so don't bother going too big - I have a 400GB that's OK, but my 1TB drive seemed to give it issues.
3. 7200rpm plus a decent amount of onboard buffer
4. Assuming you use PVR, the disk is going to be writing all the time, so if you can get a case with a fan in it that would be a good thing - my "no fan needed" aluminium case gets bloody hot, too hot to pick up some days.