Depends on your group make-up, and also you knowledge of the Shadowrun Universe.
Arsenal and the new Attitude are good suggestions for any group make-up. Gear and filling fluff is always good. Augmentation may be a good purchase as well, but isn't really necessary.
If you don't know the Shadowrun history that well, 6th World Almanac is a great purchase as it lists the history of Shadowrun in one package. If you have a good idea, then you can pick this up later.
On the old editions E-Book fronts, I'd suggest "Street Samurai Catalog", "Shadowtech", "Corporate Security Handbook", and "Fields of Fire" for gun, cybernetics, and vehicle porn (Lots of nice pictures that give a good idea of what things look like. Style matters a lot!), if you got the extra cash for pure artistic enjoyment. (Again, depends on group dynamics.).
If you want a good street-level view of Shadowrun, you can't beat the Nigel Findley Omnibus! It's 2050s rather than 2070s, but a lot of it still applies. This was my first exposure to Shadowrun (Shadowplay and 2XS) and is what sold me on the universe, and made me the raving, and possibly rabid, fanboy that I am. I just wish that I could have seen more of his work. (Pours a 40 on the curb.).
If you want to really delve into the world, I'd also suggest printing and keeping the
Slang Guidebook from the official website handy at the table. But copy/paste it into a word processor for easier reading in Dead Tree Format.
Those are just my suggestions for general items that I'd suggest to a new GM. Again, depending on your background and the dynamics of your group, this might be very different.