Staff selection is great for SMN these days. Soulscourge is a nice alternative for people that do not want to dump the time into making the ToTM staffs for Pet: mab and/or Pet: acc. Also, Kirin's Pole augmented via Byakko's Scrap can get up to Summoning Magic Skill +12. As far as Elemental staves go, get a Fay Crozier with Perp -3 or a Bahamut's staff if possible, otherwise do teh ToTM staves taht grant -6 perp.
Grip selection is terrible. Use Oneiros grip, Vox Grip, Staff strap, Vivid strap(+1) or any combination of these. Latent on Oneiros requires your base MP (includes merits and weapon MP) to be below 76%. Oneiros also grants fulltime regen effect.
Ammo selection is probably worse than grips. Hegdehog bomb, White Tathlum and Impatiens are the way to go. Astral Pot is great if you can get one from ENM60, but it's a charged item and has to be reused everytime your pet is resummoned.
Back slots are kind of limited as well for Summoner. Tiresias' cape, Astute cape and PDT/MDT-% cape like Cheviot are really the 3 best options. Realistically you can use all 3. Altius Mantle can get augments similar to the stats on Tiresias' cape (it can get 2 Pet MAB and -2 delay).
Waist selection has pretty much always been Hierarch belt until this time last year. Now we have Caller's sash (avatar mab +2), Mujin obi (enhances avatar attack), Summoning belt (+2 skill) and Diabolos rope (Blood Boon +3 and Diabolos perp -1) to use. All are great and have their specific uses. Austerity belt replaces Hierarch belt if you carry hMP stuff.