I used to have lfs. I just logged in at the website and downloaded S1 since that was the license I had. I would like to try out S2.

What files do I need? Should I install my S1 and log into game, then dl S2?

you downloaded S1? the old version 0.3h?
if that's the case, just download S2 (i.e. 0.5v) and you can join online games; there are few servers, but one is really busy that time - the STCC one
if you want to get access to s2 content, i'm afraid you have to spend £12 more

you can unlock all s1 content of the s2 version by unlocking it with your current license

have you bought s2, without it being bought you cant access s2 stuff. buy it here

edit, beat me to it

Ok so the S2 download will allow me to use my S1 license and "some" (demo) content from S2? Full S2 content comes with S2 license purchase...

Exactly.

Thanks guys...

Quote from Hedge :

Ok so the S2 download will allow me to use my S1 license and "some" (demo) content from S2? Full S2 content comes with S2 license purchase...

Well, not quite: You get everything you had in S1-times, with the improved physics, graphics and so on. Only the content that has been added with the release of S2 will be locked (obviously).