MicroSD card is a must. They're pretty cheap on Amazon right now,
The one I got back in 2017 was a SanDisk Ultra 256 GB card. I've still got plenty of space left on it and right now you can get it for $37, which is about half the price I bought it for back then. It was on sale too. Though now, I would recommend the 400 GB one which is on sale right now for $52.50.
As for accessories, a Pro Controller, while a bit on the pricey side, is well worth it I find. Much more convenient and comfortable then the standard Joy-Cons with the handle. Also, I've had my 2 Pro Controllers since 2017, one of them I got at launch on March 3rd, and they both still work great with no drifting whatsoever.
Hardware Comparison Threads:
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020