I’m not sure what the situation you describe involves (speakerphone? Why not use a headset/mike over the internet? It works great), because from what you describe, you are talking about standard MP, just with a non-public server. Before I thought you meant a LAN game (which I dont’ think they’ve quite decided upon yet: how it’ll work).
So yeah, I don’t think you’ll bump into Steam much if you don’t play MP. Just get the cheaper SP-only version, and upgrade to MP if you ever need it. Steam will be intalled, and it will patch the game, but its not going to force you to play MP 