I've been thinking about getting team fortress 2 since it's free now. The only issue is that I'll want to get the orange box at some point. My concern is that I'll have two copies of TF2 installed, one from steam, the other from the orange box. I could always uninstall one copy but I would prefer not to hassle with it. Could I choose not to download TF2 again from steam when I get the orange box? or should I just skip downloading TF2 by itself and just get the orange box now?

It won't install two copies. They should essentially be the same.

If you already have a game you won't get another copy on your account. Steam will warn you that you already have X many games in said pack, but that won't stop you from buying it.

I don't know if it will let you gift a premium TF2 account, but that's what would happen with other games.

thanks! Downloading now.