Angela, first of all, congrats for getting your first film made, and for doing it well enough for there to be interest from a distributor.
For U.S. filmmakers, it's pretty typical to have a distributor (and sometimes even a sales agent, if there's theatrical potential) for domestic distribution and another company handling sales for international distribution (especially, broadcast). Since most of your international sales will be to broadcasters, you don't need a distributor in each country. You just need one sales agent who can approach all the broadcasters at markets like the EFM in Berlin and MIP in Cannes.
The best ways to tell if this company is legit are to check out the films they have in their catalogue, check out their website to see how well they promote the films online and contact the producers of some of their films and see what their experience has been like. Did the company work hard for them and make sales? Have they been reliable in their reports and payments? Were they easy to communicate and collaborate with? Stuff like that.
I've found it pretty typical for an international sales agent (or distributor acting as one) to charge a 30 to 35% fee for their sales. I think it's generally less for pre-sales (25%?), but I'm not sure how common it is anymore for them to actively look for pre-sales.