Black panther extreme pack & shuri uniform. You have both of those characters close to t2 and their uniforms help a lot. + the rewards from the legendary battles can help out quite a bit.
Shuri for sure, maybe Archangel, Captain Marvel. Unless you already have them and just don't have them equipped.
That's 2500/3.7k crystals.
Who do you want to use the MRUT on? Do they have a uni? If not -
Cyclops' Age of Apocalypse uni turns him into a blast villain which helps with ABX (lil too late for today but hey)
Colossus' uni isn't required for any content but it's a nice upgrade on an already strong character.
Ant Man's newest uni is a nice upgrade to his current 500k one and will allow you to claim some extra legendary battle rewards
War Machine's IW uni is lifechanging (trust me, I just got it today) at any point in the game, though less relevant for you right now.
Vulture is a very strong ABX speed villain with his uni, and looks cool imo
These unis would be more of an "invest into the future" deal, and that about covers it unless you happen to want to buy a uni for your level 1s lol