1) I'm looking for a pre-written adventure (any edition is fine, I can convert, does not have to be Missions) to start a new group off with, and need suggestions.
2) Where should I go from there?
1)
Seasons 1-3 of Missions are free to download. They're 3rd edition (Season 1, Seattle) or 4th edition (Season 2, Denver; Season 3, New York), but that shouldn't be a problem.
http://www.shadowruntabletop.com/missions/downloads/If you do play Season 5 (Chicago), you'll want the (Free) Missions Season 5 Prep Files, at
DriveThruRPG, and the
Season 5 FAQ.
SRM05-01, Chasin' the Wind got 5/5 stars:
http://diehardgamefan.com/2013/10/02/tabletop-review-shadowrun-missions-5a-01-chasin-the-wind/Battleshop or
DriveThruRPG ($5.95). From there you can get the second and third adventures, which are also currently available. But Chasin' the Wind has the most reviews on DriveThruRPG.
As far as other adventures go, my first suggest is
Elven Blood. It's a 4th edition PDF, but the reason for its publication is because they were Convention Missions that were very, very well received by new and old players, especially Ancient Pawns(I think AP should be updated to 5th edition and made freely available, but I'm just a lowly freelancer).
http://diehardgamefan.com/2012/09/25/tabletop-review-shadowrun-elven-blood/Sprawl Wilds and
Firing Line are collections based on the
Elven Blood format of publishing popular convention Missions (which hadn't been in past years). The benefit they also provide in addition to being for any group, especially new players, is that they contain 5th edition rules.
2)
This depends entirely on what you and your players want to do once you've gotten some experience with the game under your belts. You can consider converting earlier modules, or you can get campaign books, adventure compilations that are more freeform and reliant on GM setup, like
Artifacts Unbound,
Corporate Intrigue, and
Jet Set. You could then, or additionally, jump into the full SR universe with plotbooks like
Storm Front (which covers several plots) or
Stolen Souls (which focuses on one plot).