Hello Nanu,
If it’s for CCIE R&S preparation, I would highly recommend Cisco’s VIRL. You’ll have the latest IOS versions and it supports L2 switching, it’s similar to what is used in the lab. Cisco has a resource calculator for VIRL.
You will use the IOSv image most of the time, that’s the Cisco IOS router. Each router requires about 512MB of RAM so that’s ~10GB of RAM for 20 routers. They also recommend 6-7 CPU cores for this. My VIRL installation uses about 40GB of hard disk space.
I would recommend to run it in VMware, you could use VMware workstation on a desktop/laptop or ESXi on a server. If you only have one computer that you work on then VMware workstation is a good idea. You might want to use an SSD drive to speed things up a bit.
If you work from multiple locations, then it might be easier to use a server so that you can remotely access your VIRL setup.
I’m using an older HP Proliant DL360 G7 server with 64GB of RAM since I work from the office, home and sometimes on the road. These servers are very affordable nowadays, for example:
That’s a machine with 8 CPU cores and 32GB of RAM in total for $399, pretty good deal.
Rene