Literally anything that gives her Health, armour, magic resistance, and ability power. Just go to the "Health" tab and basically pick out the best items, and then filter those through the enemy's team composition.
9/21/0 MS quints (haven't tried flat HP or HP regen.)/AS reds/armor yellows/MR blues (haven't tried CDR) Spirit Golem (switch w/ Locket if you don't need tenacity) > Mobi > Runic if MR is needed otherwise I go Randuins here (I completely forgot about getting Locket here but I rarely do it recently) > Liandry's > w/e you need
Rush Sunfire Cape and you can just glue yourself to the target and deal 1k damage between that and your W
Can you explain why Sunfire Cape is needed for a jungler? In my theorycrafting the 40 dps is better on a top player for farming than it is for jungling and I've heard over and over Sunfire Cape is cost inefficient otherwise.
Sunfire cape is better on Sej then most due to her w getting damage equal to 16% of her bonus hp.
Look at it like this.
450 hp and 45 armor, plus 40 burn damage for most becomes 450hp 45 armor, 40 burn, 72 extra dps for her w. the extra 72 is for 4 seconds so overall it a boost of 284 damage. I mainly build hp, armor/mr as needed, cdr and magic pen. Ap is nice but your main damage source is her w's bonus hp and her new %hp on her q.
edit: Forgot to add the sunfire's damage so the over all damage if you can stick to a target for the 4 secs for her 4 you get 284 plus 160 from sunfire for a grand total of 444 extra damage from one item that also gives hp and armor.