I see some good suggestions, and some which I find a step too far for starting summoners.
Especially for one that mentions not having done any endgame content.
First off:
Don't even start considering Skirmish gear until you're ready to take Summoner to the next level.
Yes, the gear is amazing and comes with incredible stats but it's also insanely luck based due to the random augmenting system behind it.
Getting Skirmish gear the right balance of augments can become extremely expensive and frustrating, so really save that for when you really want to take summoner to the next level.
Now for a starting Summoner you'll want the following items:
evoker's ring <- I see this one fantastic item being overlooked so much lately.
eminent pole
eminent sachet
Upgrade the Empyrian set to at least 109, this set alone already offers a really nice balance between perpetuation cost reduction, summoning magic skill as well as a little bit of extra Blood Pact Damage.
Add to this a pair of Assiduity pants (+1) and you'll cap perpetuation cost without a weapon and have an extra source of refresh in your perpetuation set.
Really basic stuff, but enough to get you started.
From here on out you can start building further with gathering and upgrading the Relic set for Blood Pact Ability delay reduction and the Artifact body if you want the extra 2% Blood pact damage.
And not to forget, the Artifact feet which will do great as Blood Pact piece until you get to Helios.
Optional items:
Gridarvor With the perpetuation reduction you get from the Beckoner's gear this isn't a vital piece but it comes with a couple of nice to have extra avatar stats.
shomonjijoe +1 Nice idle piece because of the +3 refresh but the NM can be an absolute pain.
seraphicaller Upgrade to this from Eminent Sachet as soon as you can.
conveyance cape The cape itself is easy to get, spam reives until you get this or 3 other JSE capes and trade those in for this one. Try to get a few good augments through
refractive crystals.
mujin obi and
moepapa stone
fervor ring
If you want to take Summoner even further after that I'd suggest you go back to the first page and start aiming for the sets Lunareticc posted.
But if you do, prepare to bleed gil.
As for practical endgame uses for summoner, quite a lot but to be brutally honest don't count on being picked up for those a lot.
As damage dealer the incredible damage numbers we can push require very specific party setups on which most pick ups don't burn themselves as well as gear.
It has its uses as a buffing job too, but on that department there are other jobs most pick up parties prefer to have in that spot. Like Corsairs, bards, Red Mages and Geomancers.