CCG IRL

While I'm personally heavily biased towards MTG, seeing as how I've played it for almost half my life now and write about it here on the site, I think your best option would be with finding a single player digital card game, as typically its hard to find communities to play some of the more niche card games. There's stuff like Slay the Spire and Monster Train, which I'd both rate fairly high. The other thing I would reccomend are "board game"-esque card games, where you just need to find another person to play with since all the cards come with the board, this would be stuff like Munchkin and Codename, though these types of games are for more than 2 people, so that might not be the best option.

If your dead set on finding another TCG besides MTG, I've heard generally good things about Yu-Gi-Oh!, Pokemon, Cardfight, and Dragon Ball Super. Granted I never got to play a lot of Yu-Gi-Oh!, Cardfight, and Dragon Ball Super so I'm not sure what the pro's and con's of each of them are, as well as the latter two being much smaller than the 'main three so it might be harder to fight a dedicated playgroup.

The best ways to start off would be first to just pick a way you want to play. MTG has a ton of formats all with different power levels/budgets so finding the best ones before jumping in would probably be the best. If i were to recommend something, the planeswalker decks are a great way to start. Relatively low power level, but they are designed to be played against each other in a casual setting.

Aside from that the main format I'd recommend trying out if you have some money to spend would be Commander. instead of being 2 players, there's 4 and it's a completely different dynamic to the game more focused on politics and working together to shut down a certain person if they are about to win. For these I would suggest picking up one of the commander decks WotC has released recently. There are only a few that are really iffy and all have some cool shenanigans you can do with only a 10-20 dollar upgrade. Something else to note is that despite being designed for 4 players, it is still totally possible to play with 2 players.

If your worried about investing money into these, since buying two commander decks would be somewhat on the pricier side, you can always just pick up some random cards, put them in sleeves then print out pictures of the cards and play via proxies. It's a great way to try something out and figure if you like it or not before spending money on actually making it.

If there's any other questions you have, just let me know and I'll see what I can find.

When it comes to expanding your decks, it's totally dependent on what you go with. Challenger decks were for Standard, although they have since mostly rotated out or been banned. This is to say if you do buy them, you'll most likely just be playing kitchen table magic, which basically just means you can do whatever you want, hence why you might want to set up some rules for you and your playgroup to make sure nobody exceeds a certain budget or plays an unfair deck.

As for commander decks, this once again boils down to what you want to do. Both Kitchen Table and Commander are very diverse formats, to the point where tonight I played a game where I let people cheat on casting mana and sped up the game by about 15 minutes from doing so. The next match I ended up dragging the game by 30 minutes by denying people resources and it would have gone on longer if my opponents did not decide to concede. It really boils down to finding something that you enjoy and making it work. The commander precons all provide a great out-of-box experience and for even just 10-15 dollars, you can make them much stronger or for a specific niche. For example, about a year ago I picked up the precon staring [Hearthstone Card (Ghired, Conclave Exile) Not Found], who is all about flooding the board with tokens and beating them down with it and for about 20 dollars, I was able to swap it up to instead being a combo deck based around creating then destroying eggs to flip into different creatures with [Hearthstone Card (Atla Palani, Nest Tender) Not Found].

All that being said, I understand all that stuff might be daunting, so if you want some help picking out a deck or any further questions about how to get started, just let me know and I'll get around to it when I can.

It's less about combining other commander decks and more about swapping cards out for ones that would perform better with what you are trying to do. For example, in the precon I had, it had cards that would not be great in the deck, such as Desolation Twin in it, which would be generally fairly bad. Because of that, I took out that card and added in Martyr's Cause as it would be significantly better. In Commander, the only main rules of deck building are that you cannot have cards outside of your commanders colors, so I cant run a Blue Green deck and have Red cards in it, your deck has to be a singleton deck, and it cannot include cards from the banlist listed here. Anything else is fair game unless your playgroup wants to restrict something else for being unfun. 

As for commander decks to pick from, this year has been sort of strange in regards to how they have been selling the commander decks. The last several years they have sold the commander products on their own as a set of 4-5 decks. This year, they've decided to use "themed commander" decks, so theres been two from the most recent set of Zendikar Rising and 4 from Ikoria: L:air of Behemoths. Instead of just buying the ones from Zendikar Rising, I'd probably take a look at all 6 of those since all of them should still be on the cheaper side of precons (some of the older decks have gotten very pricy). All of the decklists are also publicized online, so I'd recommend finding one that seems to do something that your interested in.

Oh and something else. If none of the precons interest you, you can usually build a decent deck for 40-50 dollars, not including lands which you can usually get for free at an LGS. A great website for just finding other commanders to build is EDHrec under their commanders dropdown.