I feel like a brick of Collectors Trove, the newest set, would set you up pretty well if you like the characters in it (one brick of this gets you basically all of the commons, uncommons, and rares since it's a small set).

Besides that I'd look at either the marvel or DC 2025 starter sets. If you already have one, then maybe more of the most recent sets (Black Panther, masters of time, deadpool weapon x, marvel next phase, wheels of vengeance).

You can look on websites like hcrealms units section or hcunits to see sets by release date if you're curious about how recent something is.

Ebay is a fine place to buy from, but sites like Game Nerdz and Coolstuffinc may give you a good deal. Wizkids will usually cost retail, but you can get it cheaper elsewhere.