Minimum IMO is a cheap B6 style charger on Amazon. These have been rebranded countless times, but they all seem okayish.
After that, I'd say save your pennies for a bit until you can get a decent dual channel charger, which is basically just two independent chargers in one box that share an interface. They're generally a better price than two chargers of the same quality, and it's really nice to be able to charge two batteries at once. When buying reputable brands, price is proportional to the charging throughput. The first one linked below, for example, is 50W per channel, and wouldn't be powerful enough to concurrently quick charge large 4S+ batteries, but it's plenty for the 2S and 3S batteries you'll likely run at 1/24 and 1/18 scale. The second one is 80W per channel, so you're paying for a bit more power.
I also recommend picking up the following:
Generally you want to store you batteries at storage voltage, or about 3.8v per cell, and then charge them up when you're going to use them. Once you've got a bunch of batteries, it's nice to have the little charge checker for when you forget which were charged, or whether you already put them back into storage charge. Your charger will have a setting called Storage Charge that will do this for you. Keeping them in a LiPo bag, inside and ammo can (with the ammo can's weather seal removed), is generally the best value fireproofing for these little batteries. Also, if your battery ever smokes, puffs up, or the cells go way out of balance, you'll want to dispose of that battery. Lots of vids on YouTube about how to do that safety, and many local hobby shops will also take bad batteries and do that for you.
For batteries I've really good luck with this brand on Amazon:
I don't run this exact pack but I run the 4s 6500 version in my XMAXX and they are great for the price. Highly recommend changing all your Traxxas connectors to XT90 down the road too.
Battery charger I am running this:
Battery disharger I am running this:
I consider this optional honestly. You can just run the car and discharge the batteries too. A discharger on the above charger also works but takes a painfully long time. I have a lot of batteries and multiple cars. I'll take a few cars out with fully charged batteries with friends and sometimes we wont go through all the packs so the discharger is nice to have and discharges the battery a lot faster.
Hope this helps!