There's only one official case for the Evercade so far, but it looks pretty decent. Once I get my hands on my unit, I'll figure out what other portable wallets or sleeves it can fit into safely.

One thing you will need is a mini-HD-out lead to connect to a HDTV, as this isn't included in either of the bundles. There are plenty of them on the market and the Amazon Basics should do you well, better than some of the cheaper, no-brand, examples.
Finally, for now, the plastic screen on the Evercade seems rather weak from initial reviews, and if it is anything like the Switch, could be prone to dings, so I'd suggest picking up a screen protector. The suggested example is for the PSP which had the same screen size, so should work. But you might want to hold off until I've checked it out.
Will update as new accessories appear or essential purchases become apparent.