iTunes content in two places. Need to consolidate.

I have my iTunes library with purchased and personal music on an old computer. I recently purchased a few pricey items (audiobooks) using my new computer. When I synced my iPod to the new computer, everything was wiped except purchases. So now I have some items on my new computer and some on the old. I thought when I connected to the iTunes store on the old computer the new purchases would download, but they didn't.

I have Match, but I don't want to just access my iTunes content. I want to download all my content to my new computer and be able to sync it to my ancient and beloved iPod. I'm thinking if I move my iTunes to the new computer I will lose the new purchases on it. The best scenario would be to have my current iTunes content on both computers, downloadable to my iPod, and to be able to access everything from my iPhone (no problems there.)

Does anyone know a way to do this? I have an external hard drive, but don't know how to use it other than a complete sync. I have used it with the old laptop but not the new. The old laptop runs Vista, the new has Windows 8...and don't even get me started on that! Suggestions anyone?