Welcome! And kudos to you for doing your research before falling in love and buying your corn snake. Speaking of research, you should get Kathy Love's book and keep it on hand. It's very thorough, easy to read, and extremely helpful. You can get it from Kathy's website,
www.cornutopia.com.
As for your current questions,
1. For most new owners, something a bit older than a hatchling is a good idea. A yearling has gotten past the delicate baby stage, is an established feeder, and is less likely to up and die for no discernible reason. An adult is fine, also. An additional advantage of not-hatchlings is that you can handle them more since they've grown past a 5-day feeding schedule.
2. Gotta go with online, unless you have a reptile show in your neck of the woods. A show gives you a chance to talk first hand to breeders, see a wide choice of animals, and get a good price. You can also find out information about the snake's genetics, in case you're interested in future breeding plans. This applies to online purchase as well. I confess, I was nervous the first time I bought a snake online. I can honestly say that I've never had a bad experience. If you go the online route, do check around (here or on other sites) and verify that the breeder whose stock you're looking at has a "good guy" reputation. There are exceptions of course, but many pet stores don't care properly for the snakes and overcharge to boot.
3. Housing is a personal preference. Small containers like shoebox tubs are convenient for small snakes. Aquaria can make beautiful vivs that are a welcome addition to decor. That one's entirely up to you. Glass is easier to heat consistently than plastic is. And hey, your choice here doesn't have to be permanent. There's nothing wrong with changing vivs as the snake grows, your space changes, you want a new look in the living room, etc.
4. F/T!!!!!! for all the reasons Sweetseraph said. It's also less expensive, which is nice. Don't you love it when the best choice is actually the cheaper one? How often does that happen? :cheers:
Addendum: There's no significant difference between males and females as far as day to day handling goes. Males are often more active in the spring and early summer when they're "looking for love." Both genders make excellent pets.
Let us know when you get your snake. Again, welcome.