Bedding is a personal preference, doesn't matter too much. For the first 10-15 years I was keeping snakes I used newspaper substrate, along with most keepers back then. It's still not a bad choice and has some advantages (cheap, easy and quick to replace entirely) and disadvantages (not terribly absorbant, most people don't like the look of it, although I actually quite liked it). I still keep a lot of snakes on paper. There are some expensive commercial options which I personally avoid because I hate spending excessive money on consumables, there's Breeders Choice which works really well and is aesthetically... a matter of personal taste. I'd suggest avoiding sand or gravel.
Enclosure size - you'll get all sorts of answers for this, but in reality, a floor space of 50cm x 40cm is comfortably adequate (10cm of height is plenty), and anything larger than that is primarily for your own viewing pleasure, to make it look pretty, etc. Many people will recommend or demand you give them large, elaborate enclosures, but it's really not necessary. That's not to say that you can't use a big enclosure if you want to. Keep it small for youngsters though, and bigger is not better; snakes, particularly young snakes, are prone to stressing in an enclosure which is too big, and relax more, feed better and thrive more in smaller enclosures. Snakes tend to thrive best in plastic tubs. They're not just convenient, they have good thermal properties and are easily cleaned. Most new keepers want something which looks better, that's up to you.
Use floor heat. Don't believe anyone who says you need UV. Don't provide any form of lighting unless you really want to for your own viewing pleasure. These are nocturnal snakes which love to spend their whole lives in caves etc. Bright light just scares them.
Get one which is already feeding reliably on thawed mice or rats. That's what you should feed it. They don't need or benefit from variety in the diet, but if you really want to amuse yourself you can occasionally feed them chunks of raw meat or a small quail or something.
Getting a small one and watching it grow is the way I like to do it too
I've almost never bought adult snakes. People tend to sell babies because they've bred them and they have to sell them. There's a tendency for people to either sell large snakes because there's a problem with them or they got bored with them and have lost interest, often have neglected them for a while and so you're buying problems. You can get lucky with large snakes, but it's less of a sure thing.
