well if you wanted to gear up iron man, doing midtown bosses with rare item find boosts active is the way to do it (while midtown madness is still active).
for stash usage, I keep all uniques. When I get 6 dupes of a unique, I keep the best one and turn in 5 either for a random unique or 500k credits - I did some turn-ins for relics and got 100 skrullos relics for my trouble. So beware that option.
I also keep most artifacts that look like they might be useful. That's a whole general tab right there. Runes and relics take up a tab by themselves, and I keep a stock of crafting mats for putting affixes on costume cores.
Cosmic rings and insignias go in my character stash tabs. Sub-60 gear I plan to use on leveling characters (level 30 cosmic medals for example) goes in that little team-up inventory window for easy access.
Costume cores are iffy. I don't keep sub-60 ones but I have regretted throwing 60 ones out in the past...particularly the 10 million m'kraan shards I got on Rocket when I first hit 60. Wish I had kept some of those for star-lord.