First of all, I can't believe it took "15"!! posts before someone commented on, "I've studied time travel", but I guess that's why you're a moderator Andail....
Down to business....I'd stay away from agencies, operatives, etc. It's being done to death right now, and although your plot may be very good, it might not be good to throw it in a mix with so many losers.
Time travelling is one of those ideas that people argue about so much, that some of the best sci-fi authors chose to forgo 2 major points in their stories:
(1 - How someone obtains the technology to travel through said 5th dimension)
(2 - Any and All quantum rules & regulations regarding their actions)
Look at Harlen Ellison, or Gene Roddenberry. Time travel is in the story, but never THE story. Don't focus on what resources I would need to build a time machine, just give him one. If the character kills Hitler, don't go back & have the world worshipping turnips, (OH NO!!! I SHOULDN'T HAVE MEDDLED). Do something like....the time machine suddenly doesn't work because the Nazis invented the Enigma machine, (aka - the first computer). Don't bother with facts, it only bores people to death.