If a pilot, can you choose to fly only on national level?

Depends on your seniority and the scheduling department.

it would depend on the company's policies, size of the company AND if the company flies internationally. I do know of one airline that is domestic only BUT every now and then the owner does secure some international contracts and some of the pilots have to fly internationally.

Pilot isn't one of the best paying jobs to start out with, in fact it sucks big time in most cases. You have to realise that you as a rookie are going to get all the crap jobs and depending on how many hours and type certifications you have you could end up not flying for a while once you are hired on. BUT you can't beat the view out the office window as a pilot.

ATC pays good to start and it is an easy job (certain locations excepted JFK comes to mind), one of the things with ATC is that you have to be able to memorise procedures and have a good grasp of vector math

I'll also be a pilot and I'd rather just fly on domestic flights.

It is also a personal issue , perhaps because time aboard an airplane is smaller.

A Pilots License IS Nation wide. It is also a lifetime license. You do have to qualify by a certain amount of take-offs and landings during a certain amount of time.