Hey Deconstruction,
Welcome to Mordheim! Sorry there isn't as much activity here about Mordheim as I remember.
Here is a link to a thread which I think should be helpful regarding thinking about how to build your warband, regardless of which warband you choose:
https://www.dakkadakka.com/dakkaforum/posts/list/713397.page
I've posted in that thread (from many moons ago), and played in my fair share of Mordheim games, but our games were just one way to play the game.
With regard to your questions about rulebooks and such. I think 80-100 GBP is outrageous for a Mordheim rulebook. I'd be patient and see if you can find one cheaper. You could also put a listing in the Trade forum here on Dakka and see if anyone has one they'd like to trade or sell. There is also a lot of activity on places like Facebook Marketplace where a lot of warhammer trading and selling has moved to.
Warbands and models. So, depending on how much of stickler you would like to be (or that your opponents are), you have a lot of options. The old Mordheim Warband sprues are like hen's teeth nowadays. I have two full sets of them on sprue, and am trying to decide whether to sell them on ebay, or keep em and use em. That being said, any era of warhammer model that you like the look of can work for a Mordheim warband - like Empire soldiers? Get 'em cheap on ebay if possible, and then use those. Pick up some character models for your Heroes, arm your henchmen all the same, and Bob's your uncle.
Or, if you don't mind going "off brand", Wargames Atlantic has Conquistadors in 28mm plastics that would work great as Empire treasure hunters in Mordheim. They also have other historical minis that in a pinch can be drafted into your band, as does Oathmark (Humans, Elves, Dwarves), Victrix (Knights, light infantry), Frostgrave, etc. After all, Mordheim is a fantasy world, and so any look, suitably "warhammered up" can work. I use alt zombies, skeletons, and vampires in my Undead Warband, for instance. And am planning to use Wargames Atlantic knights, Dark Age Irish, and some Oathmark light infantry as a Bretonnian warband. Likewise, Frostgrave Cultists work great as cultists in a Possessed Warband. The only important thing, and one of the elements we always abided by was arming the models with the correct weapons. Once you do that, so that there are no surprises, then you can use any model to proxy for an "official" Mordheim Warband.
Fortunately, there are a lot of Mordheim blogs, YT channels, and other resources that you can check out to see what the community is doing with the game. Its amazingly invigorating to see all the terrain, warbands, and such.
Undead Warbands: using
GW minis - grab some of the new zombies, ghouls, and a vampire from ebay. You only need a couple of each. Any
GW minis from the empire or bretonnia could work as Dregs in the list. You might also need a wolf or two, but I'd look to
DnD minis for those. I'm not familiar with
GWs current offerings.
Skaven Warband: again, ebay for the recent Skaven half of the starter box has very nice skaven troops. Paint one up fancier as a Hero/Leader type, and maybe grab an old metal assassin from the skaven range. Its really all what your budget allows. Slings in a skaven warband are worth their weight in gold, but I don't think there are skaven slingers available from
GW. Here is where I kitbash using a historicals kit with slingers to grab hands and slings.
Feel free to ask any other questions you might have. Good luck!