We are headed to Ecuador and Galapagos.About the only filters I'd be worrying about at that altitude would be the UV and maybe a polarizer. Where are you going on the trip?
I'm not very confident about picking out medium gray. My spot metering has been limited to: meter black, close two stops; meter white, open two stops. That and averaging out wide dynamic ranges and hope for the best.Your best metering strategy if you have a spot meter, is to pick an object that you want to render the brightness equivalent of medium gray and set your exposure accordingly. Color negative film has so much latitude that you don't need to worry too much about controlling contrast. If you are not confident in your spot metering skills, then stick to incident reading as a general practice.