If the money is an issue then my answer is definitely don't bother. None of the mods have made me think rFactor was worth buying as a mod platform, which is what it is as the standard content is pretty ropey. Even the 'official' Lienz mod is actually quite bad. I would best describe Lienz as hovercrafts with Sega rally graphics. It also has very noticeable popup, even with graphics settings on max.
You already have GTR and GTL, which are better graphically and 'feel' better, despite also using the ISI engine.
If F3 cars are your thing, I'd stick with LFS's FOX and hold out for nKpro, which looks promising.
If money is no object, then (given how much I've used it), rFactor would be a 'maybe' at best. I've tried going back to it each time a new mod or major patch was released and each time it's failed to convince me. The F3 mod was OK for a while, and Lime Rock Park is always good for a blast, but for a convincing immersive experience, LFS beats it hands down.
As for finding online games, rF is usually worse than LFS for finding a well populated race online. Despite some people's complaints (here in the USA) about LFS's lack of players, I can usually find a good race in LFS, even if it's not with a car or track combo of choice. As more mods are released for rF I think the players are spread more thinly, which is not a problem for LFS.
In short, my advice is save your cash....