The Hoard of the Dragon Queen followed by Rise of Tiamat if they start a new campaign. http://www.amazon.com/Hoard-Dragon-Queen-Adventure-Wizards/dp/0786965649/ref=sr_1_1?ie=UTF8&qid=1458181435&sr=8-1&keywords=horde+of+the+dragon+queen
It takes a lot of time to design a campaign. The one I'm running now, I started penning two years before I even met these players. And it's just a heroic tier setting, for levels one through ten.
If I were you, I'd pick up Hoard of the Dragon Queen. I think this was the first full-sized adventure for 5e, apart from the starter kit. It should have plenty of material for you guys to enjoy.
Not pdf form but it's cheap enough on amazon that you can try it and see how you enjoy it. There's also a second book to this specific campaign and between the two takes the characters from level 1(? not sure) to around 15. Also keep in mind most of these are made for 4 person parties
If you're looking for an adventure to adapt that's set in the Astral Sea there's Hunt for the Heretic in Dungeon #203 http://www.dmsguild.com/product/155343/Dungeon-203-4e
The upshot is that there's a pirate ship that's been raiding the world from the Astral Sea and the authorities want to bring that to an end. It's geared toward players around level 14 or something like that so you'll need to pull the numbers back quite a bit, but the bones of it might make for a fun adventure--you might even make whole different encounters with different, more level appropriate threats.
Another suggestion if you want to go with a published adventure would be Horde of the Dragon Queen. At the end of that adventure you end up teleporting to a somewhat unusual location to face off against the main villain. That final battle could just as well be on the Astral Sea, at which point the PCs might find themselves having that place to themselves, as in they have a stronghold (ehem and potentially a dragon friend) to enact their Skyrim fantasies as they fly around IN THE ASTRAL SEA!
Here's that one on Amazon if you're interested: https://www.amazon.com/Hoard-Dragon-Queen-Adventure-Wizards/dp/0786965649/ref=sr_1_1?ie=UTF8&qid=1506588515&sr=8-1&keywords=Horde+of+the+Dragon+Queen
But someone did outline most everything youll need for running horde and rise for "people with money".
A bit of advice among other things for new DMs here, for your specifics, though I always recommend Curse of Strahd. I'm actually running it now and it's a beautifully written adventure but it does lead to the longer side at maybe 60-100 hours depending on your party but damn, what an antagonist. You could run death House first to get a feel for how your party like that atmosphere.
I hear Deep Carbon Observatory is about that length but I've not played it myself. Maybe not so high on the RP level but you can re-work it as needed. Hoard of the Dragon Queen would be a bit over that, maybe 30 hours I think.
Looks like HOTDQ and Rise of Tiamat are also on sale for around $20:
Volo's and Xanathar's are around $30:
SCAG is $30, as is the not-yet-released Mordenkainen's Tome of Foes:
...and scrolling down the list, it seems a bunch of normally $50 adventures are also on sale around $30-34. The DMG is $32 and the Monster Manual is $34:
Are you aware of the official adventure featuring exactly that? Would be a good resource for you.
https://www.amazon.com/Hoard-Dragon-Queen-Adventure-Wizards/dp/0786965649
https://www.amazon.com/Rise-Tiamat-Adventure-Wizards-Team/dp/0786965657
Hoard of the Dragon Queen is not a free module. You can buy it on Amazon or at your FLGS.
You can pick out an Adventure (like this one and have one of your friends read through it and run the adventure for the rest of you. Adventures like that take much of the creation work off of the DM and typically provide a sort of stepping stone into play.
If you're open to trying other games:
Fiasco is a pretty good role-playing game you and your friends can play. It's much easier to learn and gives you a shot to try out role-playing with. Fiasco also lends itself very well to playing one session of it, then moving on. Most RPGs, D&D, Dungeon World and Fate included, tend to reward multi-session play.
I'm more interested in playing/running Fate Core or Fate Accelerated myself. As a game, it's more focused on narrative and player-characters doing cool stuff than D&D.
Dungeon World plays and feels similar to D&D, only it's far simpler to pick up and play. There are also a number of other games very similar to Dungeon World customized to different settings/genres, like Apocalypse World (post-apocalypse), Sprawl (Cyberpunk), Blades in the Dark (If you've ever played the video-game Dishonored, you'll see parallels in Duskwall), and probably more that I don't feel like looking up at the moment.
I'd be happy to introduce you to Fiasco or Fate, schedules willing. I've played and run both.
I have, but have never played or run Sprawl, Blades in the Dark and Dungeon World; if any of those sound like more fun and you don't mind me learning along with you, they're also an option.
While I've played and run D&D before, I don't find it fun anymore and would rather play/run other role-playing games.
For 5e, there are two "modules" out that are more like entire campaigns. Hoard of the Dragon Queen/Rise of Tiamat and Princes of the Apocalypse. Either one of those would be a great module to get your started.
You might find some on DriveThruRPG with varying levels of quality. The Goodman Games 5e modules are pretty good. I have a few of those.
> 2) how many pages could I reasonably expect to cover in 3 or 4 sessions of 4 hours each? I noticed when I was looking here[1] that the modules list number of pages, but I dint know how many to look for.
Unfortunately, that is highly variable and hard to predict. My first session with my current group burned through all my encounters about 30 minutes prior to the end of the first session. However, now that they all know each other and role-play against each other then I am lucky if we get through a couple of encounters in a 4 hour session.