If the enclosure is too big for the snake, you can compensate by adding more things to hide under, plants and such. Provided you don't have a baby snake in a 4 foot enclosure or something ridiculous. I don't think a 36x18 would be too big, provided he had cover. There are people who would disagree with that, but my snake has been a large enclosure her whole life and she is never stressed, always eats.

If money is an issue (when is it not?) you can buy silk plants at WalMart for about $1 each. You just want to clean them really well before you use them, and if they have any sharp plastic parts on the stems, you want to clip them off with nail clippers and then give them a quick file with a nail file til smooth. They won't hold up as long as the $15 plastic plants at the pet stores, but they will last months, and are a great way to provide extra cover in a large enclosure, or just make your set up more visually appealing.

I swear by the large water dish, personally. There are a lot of ball pythons who will never soak once in their life, but then there are some that will do it weekly, or right before shedding. How will you know what your snake wants without giving them the option? Mine likes to crawl through her water bowl, and right AFTER a shed, she likes to have a soak. Dunno, just what she does. They also do wonders for humidity. I always suggest them. They do no harm, and a lot of good.