I’d say you’re worrying too much about what to do at this point. You’re still in the bottleneck, where you’re more likely not to have a character to spend ISO on than to have a tough choice where to spend it.
A few bits of advice, though: You’ll only want to level that She-Hulk if you find she’s your only option among 
characters. If you keep getting her covers, then I suppose her red attack isn’t bad. But she’s very low on the list of characters you’d choose to use if you had them all. Daken and Sentry are high on that list.
Thor, Wolverine, Storm, Ares, and OBW are the gold standard
s, so you’ll want to max them, regardless. Captain America is the best
for dealing with countdown tiles.
Classic Storm, MNMagneto, and Modern Hawkeye make a super-powerful combo you’ll want to get.
Ms. Marvel and Human Torch are too new for me, but if I was starting out, and I had them, I’d level them. They suck less than Bag-Man, Bullseye, and Moonstone.
So, aside from never even keeping a Bag-Man, Bullseye, or Moonstone, I’d say level your
s when you can, and choose your
leveling strategy only after you’ve actually got the covers. It’s hard to get the HP, but I really suggest expanding your roster as much as you can, and never getting rid of a
cover. You never know which one will suddenly come your way, and you may just end up with 13 Daredevils. He’s one of the least desirable, but he wouldn’t be if he was 72 levels higher than every other character in your roster. No
sucks when they’re that far ahead. Not even Beast.
Plus, if you keep all the
s, you get more chances for more covers.
