Selecting the right SRX

I will be really greatful if some one can suggest me a SRX model based on the following:

SRX to be placed in core in a small datacenter

Total number of user is 500

 Providing:

1. Bandwidth Management
2. Web Filtering/ Application Filtering
3. Layer 7 application control (Block all kind of torrents, anonymous proxy website)
4. Gateway Antimalware
5. Gateway Antivirus
6. Intrusion Detection
7. VPN
8. Routing  (BGP is mandatory)

Kindly also let me know how did you come to choose your suggested SRX