My son is interested in getting into this game a bit. We are casual players, not for tournaments or organized play.

So aside from the Core set... what would make sense for a handful of good expansions or accessories to help get into things?

Help is appreciated!!!

first additional purchase: honestly, a second core set. This will give you additional copies of certain cards that will enhance your available decks, and enable you to build more than one deck at a time per side.

Deluxe Sets would be a good value, and the currently-releasing Flashpoint cycle. Stay up to date with current and ongoing releases, and pick & choose from the back catalogue as needed. Avoid buying packs from the Genesis and Spin cycle, as they are soon to rotate out of standard play (not that the FFG mafia is going to come to your house and take them away if you do.)

Cards you own will point the way to additional cards that will play well together. Peruse the set lists on Netrunnerdb and Cardgamedb and if you see something that looks interesting, you can pick it up or ask your FLGS to get a copy in for you. Also feel free to proxy interesting cards to see if they're worth buying.

But honestly, as of the time of this writing I would actually take your time with additional purchases. Before you get too far along, coming up within a couple of months is the Terminal Directive campaign expansion, which is directly targeted at someone in exactly your position: They own a core set, they mostly do a two-player game with it, and they want a direction to go in. That will be a terrific way to go.

I second the second core set. There are no wasted cards in an extra set. Maybe identities, but maybe not if two people intend to share the pool. Even a third set won't be wasted if both want their own slightly different decks with the same factions.

With one or more cores and the four deluxes you would have a pretty massive collection. By that time you'll know how much you like it and can decide on your own if you want to pour more money into the game

If you still want more you could start at the latest pack and go backwards.

The WC sets are a gold mine for new players.