Yes the game is worth buying - it would be worth it if it were twice the price. The answer you get here (being the LFS forum) might be a little biased, but not as much as you'd think.
How interactive pitcrew - you are the pitcrew. You decide what happens in the pitlane via F12, although some stuff is fixed, like damage repair, as it's the only way of currently penalising accidents.
No 'mods' as such, like other cars or tracks, but there are a few programs tat work with LFS to expand the content unofficially, such as a slickmod (allows all tyres on all cars [not all at once silly]), ghost mod (allows you to artificially be quick and ruins the realism of the game, but GTx fans like it), LFS Tweak (allows everyone to make silly cars that aren't drivable to demonstrate their lack of knowledge of cars).
As a demo user you should know most of this, as the only thing not available in the demo is extra cars and tracks. The features are pretty much the same (other than pit lane speed limiters, skin downloading and stats tracking).