I want to know is this a feasible idea to implement ? Are there any pit falls ? Any precautions or things to be taken care of ?

One thing that will kill you is the trigger. I have tried many scopes with similar specs and the one distinguishing factor between similar scopes is the trigger.

Sampling at 100MHZ does not make a 100MHz scope.

A 100mS A/D and a 10ns SRAM will not allow you to see some glitches that might put a '51 system out of kilter.

A Tektronix 100MHz scope does show rise and fall of a 10ns pulse i.e. this 100MHz scope samples much faster that 100MHz.

Erik