Is there much of problem purchasing a ticket after we arrive in Frankfurt? We arrive around 7 a.m. so would have all day to travel.
I suggest you reread your question and rephrase it. You are missing a lot of information for someone to begin to comprehend what advice you are looking for.
What type of train ticket are you looking to buy - where are you going, when do you intend to travel? Are you going local or long distance? etc. etc.