A reminder I used to post on other sites...when you book through an agency, with OTA or brick-and-mortar, the next step is to ask them for the airline locator number (that 6-digit string that goes on your reservation).
With that code, go to the airline's site and search for your reservation...doesn't matter if you didn't book it with them directly, you'll be able to find it with the code and your name. If not, call the airline! The reason for all this is to make sure that the agency did its job in getting you booked, and didn't make any date/destination/class errors in the process (can happen!). You want to know right away.
At that point you should also be able to choose seats, although increasingly at a price if you want to do it in advance...but in your case that might be worth it.
As for the buying: I'm basically a showroomer on Kayak and the like. I use them for research and then book with the airline, because I'd rather hear direct from them about any changes, problems, etc. Perhaps it's better now, but arriving at the airport for a flight everyone else knew would be on 5-hour delay, because they heard from the airline...