Hello, i am planning to buy cloud vps of following specifications
20gb ssd
512mb ram
1 core processor
I will have almost 30 users active at a time and 100 enrolled students.
Are these days requirements okay to run flawlessly?
Maybe so, it depends on the VPS. In general, Moodle needs more memory. If it were me, I would start with 2GB RAM. For the hard disk space, it really depends on what you and your students are doing. If you expected each student, for example, to upload a 500MB video assignment, 20GB is not enough.
With some server companies, you can buy a one-month VPS, experiment as much as you want, and then cancel. For example, when I was considering buying my current VPS, which is a 4GB VPS, I bought a one-month 2GB for experimenting. I did, then redid, then redid, and again redid my moodle installation.
In your case, you could try a 512MB server, if it doesn't seem to work, cancel and then keep moving up.
After success, I decided to buy a 4GB VPS because it was only less than $10 or so more per month.
You will find some pointers in the recent discussion "Recommended specs - new moodle installation" https://moodle.org/mod/forum/discuss.php?d=378561.