Here's the official rules and expansions:
http://www.games-workshop.com/gws/content/article.jsp?community=&catId=cat200208&categoryId=1100010&aId=5300009
And here's a site that has everything else:
http://www.mordheimer.com/
And the Town Cryer magazine plus other odds and ends:
http://tabletopgeeks.com/downloads/
Anyhow, Mordheim uses the same basic rules as
40k-sept from at basic you charge during the movement phase-, but on a smaller scale (and you control each miniature seperatly). Its easy enough to get into if you've got a couple of guys that know their way about it, but it does have the odd pernickity rule which'll do any new player's head in from time to time... (...so I...err...role doubles for critical hits now...wha?). As for warbands mercenaries are the easiest to start out with (Reiklanders usally). You'll have the largest starting warband, unless your opponent's are skaven, and still have all your men tooled up. Oh and as a rule, never take armour to start out with......it just sucks that much.
I've been playing it for a month now, but I've found that unless I'm going up against guys that are moving over to it from
40k instead of Mordheim veterans (ie every other player bar one) it can be a bit unforgiving. Its got an added degree of complexity that the newer editions of
40k lack and could certainly do with some streamlining, so if your used to just moving your troops into possition on the tabletop and letting them do their thing rather than having to go through x special rules to even get to that stage...it might not be for you.
...So yeah...personally its meh. It can be alright, but its really down to who your playing with (I felt like dropping my last opponent on his head...but he was just being a git). ^^'