Yes once you have a decent squad of 5 that you can quest with, I would suggest heavily investing in unit space. You don't have to go all out and buy gems, just upgrade here and there. I would always do it in chunks of 2 for a 10 unit increase here and there. I currently have space for 190 I think and I will fill that up when I do my 10 runs of metal parade (75ish energy so 5 runs, ding a new level, 5 more runs) and it is nice to run them all and then fuse / evolve after it is all done.
As for items I didn't upgrade them until recently simple because I am lazy and was getting tired of having to go in and sell stacks of useless shit (bugs) or forcing my self to make a bunch of items I won't use and then selling them for more gold.
Just do the quests / dailies / rewards for gems and every once and a while "buy" a guy on the summon gate that you looked into and has an "increased chance" of spawning and with whatever you have left, pick up some unit slots. With that said, I always like to keep at least 1 gem on hand for those continues when you get a really good unit on a particular run and some RNG fucked you up and wiped you.
As for how many evo mats you should keep, granted I am saying this with 190 unit slots but I only sell / fuse after I get more then 5.