You can use a normal HDD, just use the FixPartition option and it will format and partition it correctly.
It just has to be bigger, I have used a 160GB no problems when I was in a jam. It just had a lot of unallocated space.
The ARID wouldn't be an issue until after software install and licensing was needed.
If you don't manage the ARID then you will be in for a hard time to get the system to the latest release without SWAS.
The best you may be able to do is load the same version that was on there and then sync to the AMC.
Size of the hard disk is mostly irrelevant, even a 40GB or 80GB will work fine. New SSD equiped systems only use a 8GB drive.
And everyone is correct, all of this is for nothing if you do not have the Mitel Application Record ID number and know the old software version. The ARID is tied to the ID "chip" that is shipped with the system, and is assigned a software release number (4.0, 4.1, 5.0, etc) and when you bring a system online you must have the correct software version and ID module for the ARID, enter the ARID in the licensing portion of the software, then sync to Mitel's AMC. Without doing this, you will not be able to program and use any phones, voicemail, etc in the system.