They have all said it here, just get your ticket at the station, r/t is twice the o/w price. You can use a ticket kiosk or just go to the window. All employees at the stations speek English, and many on the trains do as well. Certainly take the Eurostar as it takes only 1:45 each way and is non-stop. 2nd class is fine for the trip, but if you want to spend the extra money for 1st go ahead. It should be about 50 Euros r/t and you will get an assigned seat and car when you get your ticket, no need to ask.