Other than all of that the costs of building a "full" army is relative. In my experience during a casual game I will run a 15 point list (basically a battlebox or an equivalent number of models), 25, 35, or 50 point list. Mostly it's been 35 point lists but judging by online communities and podcasts I think 50 is pretty common.
So as an exercise I used my faction (Menoth) and took it's battlebox and added the models from just the core book that I would use to expand it out to 35 points. On top of my battlebox I would have had to spend an additional $125 for what I chose. You could definitely spend alot more than that on a 35 point army, but in my opinion things are overall balanced enough to where it's not absolutely essential.
I don't know if you are at all familiar with the D&D Miniatures game, but model stats work more like that then they do 40K. A model's abilities, spells, and weaponry are static and are specifically enumerated on its stat card, unlike 40K where you have the option to swap weapons/armor/etc. per that models entry in the Codex. In that regard I think I would have preferred 40K. WM's system works great, though.
Models can be synergistic in a manner similar to cards in Magic: The Gathering. Combos can be a pretty strong factor in strategy and listbuilding. I didn't play enough 40K to see what it could do, but I was under the impression that it was not as combo-oriented as WM. In my opinion combos don't replace good playing, though. I'm sure that there are a couple retarded broken combos that even a moron could use to win, but most of them are just strong supplements to good playing.
The game itself is an assassination game. Kill the other guy's warcaster before he kills yours. The caster has a focus stat which gives him the ability to cast spells or he can allocate focus to the jacks which allows them to "boost" (add a dice to...) damage or attack rolls, make power attacks, make extra attacks, or a few other miscellaneous actions.
Attacks are made vs. a defense stat. Ties go to the attacker. A successful attack grants a damage roll. Damage is determined by rolling against armor. Damage is done by rolling over the armor stat (2d6 + attackers damage stat minus defenders armor stat = damage). Jacks will lose functionality as they take damage to specific parts of their "bodies". Warriors (non-jack models) do not.
I don't know if this answers all of your questions, but it's what I can do with my wife asking me when I'm going to be done

. If you want to know anything else just ask.