Mike, I see nothing wrong with your plan. I always like to hold off on buying tickets as long as I can (without running up the cost of things like train tickets), because you just never know what sort of delay you may encounter. Once you're in Paris, not a lot can happen, so it makes sense to take care of all those purchases at one time. Just be aware that, if you're flying in from the US that day, you will probably be really fuzzy-brained. Be sure you buy what you need and don't get befuddled about something like dates. I say that as someone who once (and not on my day of arrival, either) asked for a train ticket for a date in early August when I needed a ticket for early July.
While you're at the tourist information office, pick up other useful items. Get maps (one for each member of your party; paper maps won't last through a week of steady use), whatever brochure they have that covers opening hours at key museums, and information on special exhibits or events of interest to your group. There's likely to be something special going on that your guide book doesn't mention.
I had no problem using a Metro vending machine to buy a single ticket last year (just transiting Paris, not staying there), so I think you'll be fine if you need to buy some additional tickets later.
Ask the staffer to explain the validation process you need to follow on the Metro and on buses. That's probably the area where a tourist is likeliest to go wrong.
