If this had been a parliamentary procedure, the answer would be easy: In the chambers of governmental legislative bodies, you second motions, not notions.
However, this wasn't a parliamentary procedure, it was an email saying, "Let's buy two of monitor X." That's hardly a formal motion, and I think it could be called an idea, proposal, suggestion, or notion just as easily as it could be called a motion. So the question becomes, can you second an idea, in the same way you can second a motion at a civic meeting?
second (verb) [ trans. ]
formally support or endorse (a nomination or resolution or its proposer) as a necessary preliminary to adoption or further discussion : Bertonazzi seconded Birmingham's nomination.
• express agreement with : her view is seconded by most Indian leaders today.
Similarly, Collins lists two meanings (Nos. 18 and 20):
18. to give aid or backing to
20. to make a speech or otherwise express formal support for (a motion already proposed)
If you are using the verb second in sense 20, I would staunchly recommend using motion. However, if you are using:
I second that notion.
I support that idea.
I'd back that proposal.
then I think it's an acceptable usage (although you run the risk of something thinking you've said an eggcorn, when perhaps you haven't).
Perhaps you should simply say:
I second that idea.
to remove all doubt. If someone doesn't like that use of the verb second, you've got quite a few authors backing you up.