No, you dont need to buy additional memory for storage. the 8gb is sufficient for those.

for case choices, medyo limited compared to the Zenfone 5.
The hard case sold by Asus is good compared to others, has a bit more "coverage".
For flip cases you can check Nillkin, but check if it fits kasi the first version they made is for a slimmer Zenfone 4.
Then of course there are the generic/china tpu, etc.. cases.

Just a heads up lang, the Home, Back, and Multitask buttons sa baba ng Zenfone 4 are not illuminated. So medyo maninibago ka if its a bit dark out.

Overall, its a good phone naman, just keep in mind its entry level so don't expect it to be fast.
If you can add to your budget and be using it a lot, the Zenfone 5 might be worth it. Pero kung stick to budget talaga, this will be good enough.

There are issues of course. Some get units na medyo yellowish yung hue ng screen or may dead pixel. And I've read of 1 na may problem mic niya. Just check it as thoroughly as possible.