Before you splash out, figure out where exactly you want your hands to be. Can you get a shorter/higher stem or drop bars with a shorter reach and drop?
Try and borrow an adjustable stem to get the correct position.
If you still want a flat bar:
Note the required stem clamp diameter.
Do you want your flat bars to be straight or swept back. I switched from MTB flats to an On One Mary to get a more neutral wrist position.
You need a front shifter that is road-mech compatible. MTB shifters use different cable pull. The rear shifter is compatible between road/MTB.