This is my personal experience. Others may have different recommendations.
I think this is too small these days to run a web browser, Slack, VS Code and a couple of somewhat heavy Julia sessions. So probably not less than 16 GB?
-i5 vs i7 core?
I don’t know if this is the most important metric these days. Look at benchmarks of specific processors. There are many competing factors here, e.g. number of cores, cache sizes, clock speed, energy consumption, price, AVX instruction support, etc. A slower processor with lower energy consumption and more cores might give you a better experience than a super fast one that heats up your laptop and depletes your battery in a couple of hours. But I am no expert here. Others may have better advice.
-is 512GB SSD enough?
Depends on how much hoarding you do, how many programs you have at any one time, how big these programs are, how much data you deal with, how many backups you keep, etc. So this is very person-dependent.