If the light meter built in to the "LM" works, great, but it is limited in application at best. Still, better than none. But light meters can be had used for not much, and learning to meter with a hand held meter is a valuable skill.
A cable release can be as loing as you want it. There's no "proper" length. I'm not sure about the adapter, if needed, for a standard cable release. Could be.
Any electronic flash will work, just be sure the camera is set to the "X" setting. In fact, if the flash can be used manually with a distance scale, or on auto without input from the camera, you're good to go. You might need/want a flash bracket, though. Hanging a shoe mount flash off a tlr sideways is awkward.
Besides what else that has been mentioned: a camera bag?