Blade, Doom, Jean, Rogue (maybe, she's still in major flux from her power overhaul), Scarlet Witch, Spiderman, and War Machine are top tier, so you're pretty safe playing SW. Nothing is a must buy, although starting with Ant-man I started buying hero bundles with cash instead of using splinters since the bundle gets me a costume and a stash tab, and saves my splinters for ult upgrades and the occasional TU. TUs aren't terribly important, the difference between the best one for any given hero and the others isn't that significant. Although with the TU changes, active TUs are better than passive TUs about 90% of the time. The only time I ever use a passive TU is with Wasp on a movement hero, or Clea on a hero that benefits from spirit and/or is super spirit hungry.
Stash space is never bad but now that midtown box prestige has finally been stopped forever you don't need such vast tracts of stash space anymore. If you like costumes wait til the 50% sale comes around, although they just had one to celebrate the rebrand so it's possible there won't be another 50% sale til Halloween. I dumped 50 bucks on costumes during Halloween and don't regret it even slightly.
Emma's re-review looks amazing, even if she's tuned poorly or it's only three quarter-assed like Cable, it will be a massive improvement. I hope Nightcrawler's re-review is solid too, although it will likely be March before we even see a preview of it.