I am not sure but every 6 weeks may be a little too little. I have never heard of a ball python being offered less then 2 weeks. If he refuses then it is so, but I think he should be offered food more often. Ball pythons do not need a light or UV lighting at all. Light can actually stress them out and ball pythons do not make good decorative animals to even need a light. If the room has enough natural light then a light is not needed. Ball pythons hide for a good portion of the day, making the UV lighting completely useless.
Things to look for before purchasing, especially if this ball python is coming from an owner and not a breeder would be any health issues. Mouth looks nice and pink. Scales are healthy. Check for scaring especially along the belly. Listen to his breathing and make sure there are no popping noises. Check for mites (on the snake, in the heat pits and you may see it in the bedding) and check that the owner has kept his cage clean.
Ball pythons need to have belly heat, which means you will need a heat pad. I am not sure what you mean by "heater" whether that is a ceramic bulb heater or pad but you need the heat pad regardless. Along with a heat pad you will need some sort of thermostat. There are many offered on amazon. The more accurate ones will be more expensive.
Being eighteen years old, he is definitely not old. He could live up to 30+ years so be sure you understand that even though he is older, it will still be a very long commitment. Good luck with your new friend and post pictures once and if you get him.
Ball pythons dont need belly heat had one for a year always on ceramic no heat mat never missed a feed in brilliant health, in the wild heat comes from the sun which is above not below