Unless you've got the trading power for the aura granting rune word stuff, I'd say a summonancer's biggest concern is getting as many +skills as possible while getting the resists up. Full Trang's is good for resists, but not as good for +skillage. I'd go with three pieces of Trang's; Armor, Gloves, and Belt. This will give you great resists to Cold, Lightning, and Poison, +2 Summon, +2 Curses, immunity to freeze, and a host of other useful mods while leaving slots open for a lot more +skillage.
For Shields, Spirit is fabulous for its huge FHR and further resists to Cold, Lightning, and Poison as well as its big bonus against ranged physical attacks. Homonc is probably more ideal for resists and the extra curse points.
Shako is the easiest +2 helmet to get and fills out life and mana. Few other choices make as much sense as this one.
Ideal amu is a +2 all necro or +3 summoning with resists (particularly fire, since none of your other gear covers this) and any other decent mods.
Rings: Outside of the very rare +1 all skill rings, I'd say use rings to fill out resists or other needs.
AoKL is the only weapon choice for straight Summon Necros unless using Beast with it for a swapout to prebuff minions with.
Boots: Marrows are the obvious choice, but summoners can make it without the synergized bone armor. However, the +1 or 2 mastery is nice and won't come from any other boot. I'd say, consider other boots if you're having problems with resists or anything else. Silk Weaves are nice for recouping mana from corpse explodes and they give a hefty bonus against ranged attacks as well. War Trav's can have a huge MF bonus. There are also hotspurs for a huge fire resist bonus and Tearhaunch for +10 resist all (no other boot gives res. to all).
Charms: You're probably going to need some fire charms since none of the Trang's stuff does anything for fire resists.
Alternatives to Trang's:
Armor: Vipermagi for more resists. Skullder's for MF. Que Hegan's for nice Def Value at minimal Str reqs.
Gloves: Chance Guards for MF. Frostburns to add a hefty 40% bonus to your mana reserves. Bramble Mitts for the Huge fire resist. Death's Disguise (w/belt) for resists.
Belts: Ik's, Credendum, or Death's Disguise belt (with gloves) for resists. Goldwrap for MF.