The datasheet provides general guidelines, but a good rule of thumb is to keep the analog and digital grounds separate, and to place the device close to the analog signal sources to minimize noise. A 4-layer PCB with a solid ground plane is recommended.
Use a low-pass filter at the input, and consider adding a shield around the device to reduce electromagnetic interference (EMI). Also, use a low-ESR capacitor for the VCC pin to reduce noise.
The maximum sampling rate is 1.5Msps, but this can be limited by the analog input bandwidth and the system clock frequency. Consult the datasheet for specific timing diagrams and requirements.
No, the LTC2449CUHF is specified for 3.3V or 2.5V operation. Using a 5V power supply may damage the device or affect its performance.
The digital output is a 24-bit parallel data bus. You'll need to use a microcontroller or FPGA to capture and process the data. Consider using a FIFO or buffer to handle the data transfer.