First, buy the biggest container (not plant) you can carry. The
container the more flexible you can be with watering and care. Half
wine barrel with extra holes in bottom is a good size. Or a pot like this one that is used for hiding your hose...
We usually use cactus mix for the filler. If you have a native area nearby a couple of handfuls of 'real' dirt in the middle would help make the mix more normal.
Before you plant, water the mix a couple of times. If it's muddy feeling add some perlite, if it seems too light, add some 'native dirt'. The mix should feel like dirt you want to grow stuff in. A moist, not wet, gallon pot of the stuff should weigh between 3-4 lbs.