Hard disks for Ds-7608NI-I2/8p

Hello all,

i have a quick question. I will buy the 7608NI-I2 (8ch) and I see that it has 2 SATA slots. I was wondering what HDD I should buy for it... I see that WD makes the Purple series that is for Surveillance equipment. Is this the correct one to use?

Also what size in TB should they be? I don't want recording 24/7, just when alarms set off on one of the cameras (which will probably be the DS-2CD2342WD-I ones).

Thanks!