1) It's likely that all the barrels are being used for food storage. Make sure that you have several reserved barrels (open the stock[p]ile menu, then look near the bottom), as reserved barrels will be saved for booze.
2)Above ground, plants seem to grow wherever dirt exists on the layer below them (just my observation, may be hearsay). Below the ground, I know that towercaps will only grow if you've found an underground river. Might be the same for other plants.
3) Don't worry about the caravan right away. Check your depot pathing - hit [D] and see if the depot is marked as "accessible". If it is, you won't even need a road - and it'll usually be accessible with a bit of tree chopping, and maybe a touch of boulder smoothing.
They'll likely show up at a point where they can reach the depot.
4) Fortifications can be built above ground (although, keep in mind that you want to uild -> [C]onstruction -> [F]ortification, you can't carve fortifications into rewalled walls). They're useful mainly when you made your fort in undwarvenly territory - read, forests, plains, etc. Since your fort will start at a downward stairway, building an above-ground fort is a great way to deal with sieges and attacks.
Keep in mind, marksdwarves can't shoot up or down z-levels. Again from observation, they'll try to shoot downward, but the shot will travel straight, causing them to waste ammo shooting air. This is more of a problem above ground, where you might actually have defenses a level above the ground. Just something to keep in mind.5) Yeah, it's been answered... but keep in mind that the cave-in code will be implemented someday, so think twice before hollowing out half the mountain, or your fort may be rudely destroyed some day in the future by a massive collapse. I tend to follow the old 7x7 rule, more out of habit than as a result of actual cave-in threat. We'll have to see what Toady comes up with.
EDIT: Gah, beaten to the punch >.<
[ November 08, 2007: Message edited by: Thexor ]