There are no more "S1" servers - you can access any server that presents S1 contents (track, cars) combination.
There used to be a guaranteed S1 content only server, active, with the CTRA area - extinct.
Problem with S1 is tracks quantity - even a server running road cars (S1) like to venture on Westhill / Aston, therefore excluding S1 users
I suggest you get S2, S1 IMO is not worth it - not in term of contents, but in terms of online racing accessibility.