For opposition, I can’t recommend the Burning Rogues section in the back of the book for NPCs enough. I re-read it in Gold, having more-or-less forgotten the specifics in Revised, and promptly kicked myself. The rules are simple, smart and quick. Remember that the players are often working together. Don’t fret too much about ‘appropriate’ power levels. Use the Burning Rogues rules and you’ll be fine. So will your players. Maybe bruised or injured, but they’ll be smarter about when to run away, and they’ll have some nice Field Dressing, Herbalism and Health tests, or they’ll have been persuaded to do something really dangerous–it’s win-win!
If you’re really worried about whether your threat is absurd (or your players are whining a lot), compare your bad guys’ Mark results with your players’ PTGS. If a mark is mortal to everyone, you are telling your players to run away all the time, and that may not be what you (or they) want. Opposition should ideally present some choices, but if “Fly, you fools!” is the only way they’ll get to live, and they’re not all playing sneak-thieves and cowards, then you’re being a dick. Similarly, if your players dispatch all your goons in one exchange, you need better goons.
A large part of danger in BW is about tactics in the extended combat mechanics, not just absolute numerical power levels. A single thug is probably no match for the group. A gang, with a thug for each player–and two for the guy with armour–is much more dangerous. A gang with a weapon skill, Brawling and some good working knowledge of Fight mechanics (i.e. how to handle a guy in armor, or a fast guy with throwing knives, or a sorcerer, etc.), is more dangerous still.
Remember also to challenge your players with opposition they don’t want to kill, because of beliefs, instincts, traits, reputations or affiliations. You want to be a hero? How heroic is an adventurer who beheads a lame, impoverished 12-year old girl thief? Not very.
Also review the “Rules of Thumb in Character Burning” (pp.104-5) for player power levels. The advice on exponent caps is just as salient as limiting the number of Lifepaths, which varies by stock, play style, character concept etc. Starting a group of Men with 3 or 4 LPs is pretty safe. Most concepts can be realized in 4, easily, 3 with a bit of work. I don’t recommend 2 LPs until you’ve played a bit, but they are awesome (lots of failure, then one day ass-kicking ensues). Similarly, I wouldn’t recommend starting out with 5 or 6 LP characters until your players have experienced learning and advancing their skills, investing in new wises, and all that entertaining part of the game.
TL;DR: use Burning Rogues for opposition, ignore power disparities, they are who they are, no dragons. Start new players with 3 or 4 LPs, re-read “Rules of Thumb…”