@TotalEklypse: If I were you I would give Trackmania Nations Forever a shot (the free one on Steam) as that should give you a good idea of how they all play. As far as I am aware none of the Trackmania games include crashes with other cars, the only thing you can collide with are the walls (and when that happens on Trackmania 2 you see some visible car damage).
@TotalEklypse: Can't psuh other players off i'm afraid. Also I cant seem to find a Trackmania 2 page on steam, are you perhaps looking at "United Forever"? as that is a different older game. as far as i'm aware there is nothing on Steam for this game at this time and it can only be purchased through the trackmania website.
@TotalEklypse: It does not work on Steam at this time, and from this thread on NeoGaf http://www.neogaf.com/forum/showthread.php?t=440418 it does not seem like it will be in the immediate future, however the developers seems to want to ensure that anyone that buys it now will also get a Steam copy when and if it appears on there (so they don't have to buy it twice). Not sure that this would actually happen though.
As for the crashes I can't really say as I am busy downloading it now and haven't played it yet, however if it is anything like the others (which I believe it is). You can't crash into other cars as they are ghosts, it's more about a time trial type race on crazy tracks more than anything else.