Board Thread:General Discussion/@comment-11700680-20170123123344/@comment-217.10.52.10-20170123143016

I asked the same question two months ago (15th November), and you can find a lot of valuable information and links in the responses I got in the Akashi I.A. comment section http://kancolle.wikia.com/wiki/Akashi%27s_Improvement_Arsenal#WikiaArticleComments

Basically some equipment like T91 shells and ASW gear are fairly cheap in screws and resources yet valuable, and then also some type 21 fighters as they maintain the upgrade level as you convert them to new types by various quests. But better check the details over there, or wait for the experts to give you a more detailed response ;o)