This is a stumbling block most new players are unprepared for. Building your roster takes time and patience.
First I want to mention that in the long run, you will want to roster all two-star characters as they will all be required for PvE sooner or later, and the champion rewards are good, especially when you are in the Three-star phase and can sell off and reroster your twostars for more champion rewards (same goes for higher rarities eventually by the way). Don't do that while you still rely on your two-stars for Daily play though.
You should keep Iron Man * on your roster just so you can do the DDQ one-star node each day. Every Little bit of ISO helps.
On the two-star front, I suggest you roster at the very least Daken, Black Widow and Ares. All Three are excellent and form the backbone of two-star play. Hawkeye and Human Torch are nice too, but not quite essential IMO. Moonstone, Steve Rogers and Bullseye are low priority.
On the Three-star front, I agree with the Cuts you suggested. Captain Marvel can probably be cut too, her main use is to generate red and black for other characters (at least when I use her). I'd probably try to roster Iron Man and Iron Fist at the very least, they are both at the top of the pile in Three-star land. Scarlet Witch is also very good, and you might want to roster Hawkeye just because you have a lot more covers for him.
Also, some general advice: Only use your HP for roster spots, nothing else until you have all the roster spots you need (I am up over 150 spots and I still need more...). Don't open tokens unless you can afford Another roster spot - when you have enough HP to expand your roster, then you can start opening tokens until you run out of roster spots again. And finally, don't waste your ISO on your Three-star characters right now, use it on your two-stars.