Why I choose RS232 standard means, there would be loss of data at 8KB/S in future system. In existing system speed rate was nearer to 70B/S. Will the existing system(IRF540+ULN2003) is enough to carry data without loss?
Seventy bits per second (70B/S) is a very slow (and non-standard) data rate. Even 8000 bits per second (8KB/S) is considered a very slow (and non-standard) data rate. Commonly used baud rates for RS232 are 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bits per second. All of these are faster than your 70 bits per second, and all 9600 and up are faster than your 8000 bits per second. That DOES NOT MEAN that you NEED an RS-232 data link!
If you are losing "data" with your existing low-performance system there is something wrong with it's design, perhaps a fault in your signaling integrity... for example too slow of a transition on data edges, not enough setup time before clocking data into a register, excessive timing jitter between the data and the clock signal, noise on the data signal lines, lack of hysteresis in the data inputs to the receiver... yada, yada, yada. All of these things can be measured, quantified, and compared against device specifications. Sometimes just looking at the data signal line with an oscilloscope triggered from the clock line will reveal where the problem lies. It is necessary to have adequate instrumentation BEFORE attempting to design, much less build, a reliable data communications link. What tools do you you currently possess? Can you obtain the tools you need?
Clock, data and other control signals are came along with data lines. I'm replacing transistor driver with RS232 because I already said there might be data loss in increasing data rate and this is my doubt.
"Might be data loss in increasing data rate" is not sufficient reason to replace the mosfet drivers in the existing system with RS-232 bi-polar drivers. On what facts and information do you base your doubt? You need to examine the signal integrity of the existing system and determine why
it isn't performing as expected. A single-ended, high-level, serial data stream should easily communicate error-free data at speeds exceeding 9,600 bits per second. Find out what it wrong before attempting to "fix" it with technology you do not yet understand.