(.....)
In terms on online server, amazon instances are pretty good (we run both an BTC and ETH explorer) which are accessed from anywhere in the world using RPC calls and runs 24/7. Cost is about 300 USD/month for the data transfer.
This is quite a huge amount of money since the OP said he/she wanted to gain personal experience of developing blockchain explorer.
Do you have any idea if, for example, I'll do it first locally, just for fun or experience just what like OP wants, Do I still need those 500GB of space
or minimum/DELL R200 with 4GB of ram and an old Pentium CPU just what you told?
The only difference to run it locally would be to skip the Amazon EC2 instance costs.
If you are on a budget there are several GITHUB repositories you could borrow from but it is still quite a challenging project
to run your own block explorer. (instead of hiring an experienced coder)