Yes, as Nala said, welcome!
I say get your hands on the living rulebook, then tweak using your own judgement as you go. Main tweaks are often cnoncerning "Dual Wielding", weapon tweaks, henchman groups and making skills or rules suck less
You can definately use you old WH figs! In fact, the Empire MIlitia set were the basic Mordheim sprue you got back in the days when you could buy the box! You can build any and all sorts of henchmen from that sprue alone (at least for human mercenary warbands). Then you can convert or get your hands on some more spectacular models for your heroes.
Also, be sure to download all the issues of Town Cryer and the rulebook.
A lot can be found here:
(i took the liberty of posting a link, Svenn
Get the rulebook and other goodies here:
(its GW official site)
Anyway, in any pother case, you can just browse the forums and see what we discuss about the game here. A lot of people use their own house rules, best is to search for them (they pop up frequently, such as topics about slings, spears or dual wielding... and watch out for getting knee deep into discussions about powergaming
Anything else, done hesitate to ask!