Cory, I have programmed 3 different brands of PLCs and none of them were interpreters.
Manjeet, the EASY part is the PLC engine, the difficult part is the ladder logic editor/compiler. In my PLC work I have found no problems in the PLC units, but bountiful in the editors/compilers and based on that and my knowledge I make the above statement.
If you are not out there to compete with Siemens, Fanuc, Allen-Bradley, IDEC, keyence and others but trying to make a control system for a specific purpose you will be better off making it as a regular 8051 control system.
anyhow, have fun
Erik