Hey Olly, In reality, you want at least same speed HDD and same or bigger hard drive to work correctly, If they cutting on your budget i would potentially look into refurb hdd
Here are information you need to find hard drive you need
Option Part# 627117-B21
Spare Part# 627195-001
Assembly Part# 627114-002
Model# EH0300FBQDD
Base on spare part Nr you can find compatible
I hope this will help