Don’t worry about the deck’s cooling, the built in fan is fine. It will work fine with a case as well as long as the case doesn’t cover the vents.

It’s possible to get by with a 64gb deck and microSD cards, but it can be difficult. Even if you don’t install any games to the internal storage, things like shader cache will still be saved to the internal storage and can fill it up. A faster, larger SSD is really nice. Opening the deck isn’t very hard (just make sure you use a proper size screwdriver, make sure it’s fully inserted in the screws before turning them, and don’t overtighten when you put them back).