PC specifications depends on your requirement. If it doesn't matter we recommend to use Windows (10 version) because most of our customers use Windows version and we are often release some intermediate versions for this platform with customer feature requests. Minimum hardware requirements here: teslascada.com/HTML/overview.html
If your project is big enough - contains 1000 tags and more, and you have many images in your project we recommend to increase RAM up to 4GB. If you use Web-server its better to use 8GB. I think all modern CPU and graphic cards will work with TeslaSCADA2 without any problems. 128 GB (minimum selling) disk will be enough for your purposes.