I have a little different take on it.
First,get some training.First responder or EMT.You need to know what to do,and what not to do.
From there,suppose you want to be ready for a gunshot wound at the range.
Boom.
Best thing you can do,get an ambulance and a couple of paramedics rolling toward the patient,flight for life if you are remote.Make the call.
Next,don't worry about transporting the patient,or preparing the patient for transport.You just keep them alive,hopefully.
Breathing is good.If they stop,you may have to help them.CPR class,maybe a pocket mask.Installing airways takes training.
You may need to know about pneumothorax,sucking chest wound.A hole letting air into the chest cavity will defeat the mechanics of breathing.For a while,a hand can seal it off.
Bleeding.Bleeding is bad.I have heard Quick Clot is good,but have no training or experience.
My local surplus store sells GI Carlisle type battle dressings.I have the standard 4 in by 7,I think,and a larger 7 by 9,maybe.The plastic wrapper may be useful for sealing a chest wound.Maybe feminine tampons or napkins would be useful in a pinch.
I do not know if they are approved,but large ace bandages can apply pressure over those dressings.So can your hand.
Yes,a tourniquet might be the thing on an extremity.You might kill the limb,though.
A shattered long bone might need some traction,as muscle contractions can be a problem.
But picking up your patient,throwing him in the Bronco,and blasting down a washboard gravel road 90 mph is more likely to kill him than just keeping him still and stable where he is.
I don't know that $500 in first aid supplies will make much difference.