I know Dell has a good presence down/under. Look for a system using the Windows SBS server. That will have your OS, SQL, email, IIS and Hyper-V. Only downside is 2008R2 isn't available in an SBS flavor yet.
If you are going to be using Hyper-V as you indicate, you will want more than 4GB of RAM, especially if you plan on allocating the majority of it towards SQL operations. But do you really need that much for the SQL DB?
By 3 drives for mirroring and Parity, do you rally mean going RAID5?
Is your $2500AU budget for hardware only or software as well? Because 2008R2, plus enough CALs for your client base plus SQL is almost that amount right there. 2008R2 Standard includes 5 CALs. Enterprise includes 10. But Enterprise is $3000US right there. Standard is $800US.