Which engine and ROM should my bot have?

Every chassis comes with the small engine, ENG-S. That's fine for a light bot, but a loaded medium or heavy chassis needs ENG-M or ENG-L to stay quick. The ROM (ROM-S, ROM-M or ROM-L) only sets how long your program can be. It doesn't give your bot more speed or thinking time, so keep ROM-S until your program outgrows it.