As far as the expiration or use by dates, if the item in question is something like a gauze pad or a triangular bandage, I'd go ahead and keep it. If it's medicinal, toss it out. Some bandages can work long after their date but the packaging doesn't, you'll find that the glue used to hold them together will most likely be dried up and whatever sterility was there will be gone.

You can pick up a box of 100 single sized Rite Aid brand for about 6 bucks at the moment, should be plenty to stock up all of your kits.

Other than that, remember your ABC's, in a true first aid/trauma situation you'll want to be able to keep an airway open, provide respirations and stop bleeding. That's really all that a lay person should be doing anyway.

I'd focus on that first and then add some "nice to have" stuff afterwards. ie, personal meds, splinter tweezers, etc.