Do you want it to work reliably, or do you want it to be cheap ?

Is your time valuable ? If you can only use the chip you have, where/how will you get the support chips to do the programming ?

The AT89C does not have ISP or IAP, which will make programming harder.

It's much easier to buy a part with built-in In Application Programming, that can be programmed through a standard PC serial port, such as P89C family (Philips), or T89C51RD family (atmel-wireless). Both also have PWM, useful for motor control.