Hi Kumar,
I might be missing something important, but won't the AD7760 provide this? The AD7679 provides 18-bits at 570 kSPS, while the AD7760 provides 24-bits at 2.5 MSPS. Admittedly, I haven't done an exhaustive analysis on this, but wanted to make sure I understood any limitations you had already noted.
Best,
NevadaMark