Recommendations for AutoCAD add ons electrical and plumbing

What is your recommendation for a good add on to do electrical schematics?
I need one that has all the symbols, too.
What program do you think is best to use for piping schematics? I will also
need to do isometric schematics and need all the appropriate symbols.
I do not want to spend any more money than necessary but would like to get a
quality piece of software.
Thanks in advance,
Carolyn