Just use a normal mortice latch, basically all these townhouses as they're called (3 storey) that are always built these days ,virtually all the doors inside these houses have to be fire door (apart from cuboards etc) then some of those have to be as well, but they still get the standard latch and handles that are fitted to the rest of the doors, be careful with paying for those hinges as well, I don't ever remember needing special hinges either , basically a pair and a half (3) quality 4" hinges are all that are required to basically stop the door bending and to take the extra weight, if you are fitting it as a fire door, they are usually required to be self closing as well, this is usually done by fitting a Perko, or similar named device
If you need it to lock thats no problem, just fit a dead lock or a mortice lock, and don't forget the door brushes, there supposed to be on there as well , both sides and the head, either in the frame or on the door, yes there a pain in the a**e but should be fitted to a fire door.
If it hasn't got to be a real pretty door or just painted, we used to hang personel doors between the garage and the internal house that are basically just painted door blanks (plain flat both sides), these were very light but still fire rated, they were basically a hollow door filled with what looked like already burnt materials? But they really were light compared to these new internal fire doors (same pattern as any door you want).
Also what fire rating you want will help decide on the door weight factor, a 1hr door will be heavier than a 1/2 hr fire door