How much Paint

I have a few questions on paint. I have decided on either this paint kit or this paint kit by TCP global. I am not sure the difference to be honest, but I emailed them to find out.

My question is how much paint would I need for a 70 dart engine bay, jams, under hood and under trunk? I don't mind buying the gallon, but the rest of the car isn't going to be shot for sometime due to body work needing to be done.

If I bought the gallon will the leftover unmixed paint be OK to store, if so how long?

Thanks, everyone!