Yes, the 5512 is a good replacement for the Pix. You cannot load the Pix configuration file directly on the 5512 though. You are better off recreating the configuration from scratch manually on the 5512 and using the opportunity to vaildate all your rules.
The SSD models are only necessary if you want to use the Next Generation Firewall features like Web Security Essentials (WSE), Application Visibility and Control (AVC) or Next Generation IPS. Those all require a separate subscription license in addition to the Solid State Drive (SSD) hardware option.