I'd say go for S2 and see how it feels. The jump S0 (demo) --> S2 is huge but S2 --> S3 is relatively minor. If you really love S2, then you can go S2 --> S3. Don't worry, you will not pay more by upgrading from S2 to S3 later on because each license step costs 12 pounds (might be cheaper in Czech Republic?).
I'm saying this because most of the racing is done on S2 servers. Only sometimes you need S3 to play online at some servers. Of course you won't get Rockingham without S3 so if you really do want to try RO, then go for S3.