One thing that I don't think was mentioned that is something that I personally would like is a bomb ring for the HP Boost. Ironically we spend so much effort trying to boost our MP that we fail to consider HP. It is bad enough that in some events where I might die in battle and be raised.. any, and I mean any aoe attack will insta kill me. Granted, I wear way too much convert gear and should change that, but the added 75 HP could be helpful as I can always remove convert gear when nessessary.
I advocate both - prep items and - bp items. Ironically I bought all HQ staves back when they were twice the price they are now, but have yet to buy penance hat that has also lowered a lot in price. I think that largely has to do with the the way that I play summoner. I often solo, and thus being able to keep my avatar out as long as possible can be a greater asset. In a sense, my theory is where as lowering the BP timer I might be able to get my bps out faster, but without the mp there to use the BP, its not going to do me any good. Again, it really is case by case on what activities you do along with sense of balance. I'd recommend all of the gear mentioned in this thread, although understand you don't have to have all of it to be effective.
Most of the items are very rare and difficult to come by/and or expensive. Pace yourself, and make your goals reasonable. I.e. if you can't afford a full set of HQ staves get all of your NQ and slowly replace them one at a time. Also, do a lot of dynamis >.> hehe. You don't "need" something like a serket ring but of course depending on the sittuation it can be nice to have. I don't have one, nor do I have a hedgehog bomb. I have my old, trusty fortune egg!
Doesn't set me back at all, so again, pace yourself and make goals. Odd thing that I find with summoner is if you enjoy the job you will play it well regardless of what is filling your equipment slots. Adapt.