I can't comment on off road racing (not really my thang), but I can vouch for HistorX being absolutely glorious. Seriously - where has this mod been all my life?
I have known about it by name/reputation for quite a while, but I have just now gotten around to installing & configuring it over the last week...it has truly been a "WOW!" experience. I had been logging a lot of laps in GTL and thought I finally get around to checking out HistorX while I was in a 60's/70's mood...I've been totally blown away by it.
I love pushing the old TC's/GT's around in GTL - I felt that the feeling they captured of pushing a car to the point where you are four-wheel sliding right to the edge of the curbing on track out was just second to none...until I tried HistorX. If you had asked me if an rFactor mod (which, in my experience, can be pretty hit or miss) could do it better, I would have laughed in your face. Well, I have to tell you, HistorX physics (for my tastes) > GTL.
I still love GTL - it was one of the first true racing sims I ever played - but if I had to pick one or the other, it would be HistorX. Right off the bat, you notice how professionally done the mod is (more like a total conversion than a mod) and then of course you'll notice how beautifully modeled the cars are...but the real magic kicks in once you start pushing for laptime and realize the physics are absolutely brilliant. If you watch some vintage highlights of TC/GT racing, you can see how these cars behaved on track - relative to modern cars, a lot more body roll and also much more pronounced sliding. The HistorX physics captures this feeling wonderfully - you really feel in control of the sliding (which, in my experience, is difficult to replicate in rFactor, as I think the underlying engine seems prone to "uncatchable slides").
Sorry for going on a bit, but I've just really been impressed with the sim and I wanted to put in my 2 cents - I think rFactor might possibly be "worth it" solely for HistorX!
Really the only downside I can say is that I just don't think rFactor AI is any good. I would love to geek out on some offline fantasy seasons (the HistorX team has included tons of different championship classes complete with links to the necessary tracks in rfactorcentral), but the AI just drives me nutty (even plugging in recommended AI tuning tweaks). The AI is good enough to give you some traffic to battle through for a 5 lapper fun run, but anything beyond that and they just start to drive me crazy.
My goal is to get comfortable enough in the sim to jump in on some of the Sunday club action (after I get my "Premium" membership set up).