My step-son lives with us and his mom's not very involved, either. I'm sure this isn't what you want to hear, but my best advice to you is to move out. Even if the man himself is the perfect match for you, you can't marry a father without also marrying his kids and all the issues surrounding them. If those issues are already driving you crazy; making you hesitate to properly parent your own daughter (because it doesn't seem fair to give her harsher expectations than her step-siblings); and making you consider moving out...none of that will get better, as all the kids head toward their teens and get even more challenging to parent. If you and he are meant to be together, you'll still be meant for each other after your kids are grown. I know that sounds harsh - not to mention completely unromantic - but it's really not the rest of your lives.
My husband and I did not have any issues over discipline, when we got married. In fact, I believed he and I practically shared a mind, in terms of our expectations and priorities, regarding children. But a lot of my beliefs were based on talk. He didn't get custody of his son until shortly after we were married and before that, his ex-wife did everything she could to keep them apart. When I did get to see my husband actual parent his son, it was always during vacation times and the two of them were just so happy to see each other that there weren't many discipline issues.
Only after his son moved in with us, got comfortable and let his hair down did either of us see what issues he really had (lazy work habits, pathological lying, petty theft and sneakiness, to name several). Certainly, he's not a problem all the time and sometimes he can be just delightful. But the older he gets, the more serious these problems are, when they arise.
When my husband and I are able to talk objectively about problems with his son, we still see eye-to-eye. But I have learned that he has a powerful aversion to admitting that his beloved child has any of these problems (perhaps because they remind him so much, of his ex-wife). Many times DH has lashed out at me - even, on occasion, my older sons - to divert attention from something his son has done; or he may make irrational/irrelevant equivocations between things my sons or I do, to excuse something his son has done. Not only is this very hurtful, in the moment, but it creates a maddening quandary for me:
1- It seems so unfair to hold my sons to expectations, when they see their step-brother is not held to similar ones. Yet, I only have this one chance to parent my sons. I can't wait to do it right, until after their step-brother is grown and gone...because they'll be grown and gone, too. I can't justify slacking off in MY parenting, every time I think my husband has slacked off in HIS.
So there are double standards in our home. Reluctantly, I've even gotten to the point of acknowledging them; telling my biological sons: "I know it's not fair that I'm holding you to this standard...and your step-dad and I SAY that we expect it of ALL of you...but your step-brother gets off the hook. But I'm not his mother. I've decided it's not worth sacrificing my marriage, fighting with his dad over it anymore. And I truly believe holding you to this standard will help you be the best people you can be. So I expect this of you, whether your step-brother has to do it, or not."
2- I like to know I'm doing the right thing. But what's right? To sacrifice my own ego; remind myself this isn't my child; and bite my tongue while his parents parent him, even when I think they're screwing it up? Or...when DSS misbehaves and his mom sympathizes with and praises him (not because that's best for him, but because it's her surest route to being his favorite parent); and his dad looks the other way (because he can't stand to admit the ugliness in his son's behavior)...is it my job - as the only clear-headed, objective adult in this kid's family - to step in and try to discipline him (or at least hound his dad until HE disciplines him)? Isn't it unloving, to facilitate DSS growing into a self-centered, off-putting adult, because everyone around him (including me) has their motives for not intervening? Or should a loving step-mother put all her effort into trying to accept him as he is?
I can't say I wish I never married my husband. It's just too complicated, especially since we have a child together. Plus, my husband and I really are like-minded. He is just weak sometimes, in following through with his son...and I don't have the same authority with that son, as I do with my own.
But the problems I've described make me feel like we're two families sharing a roof...which is not what I ever wanted or expected, from blending our particular families. This makes me feel less close to my husband...and that's deeply sad for me, because I love him and at one time I didn't think it was possible for two people to be closer than we were. I also question whether I've done the right thing for my older kids. They don't feel as close to their step-dad as they used to. And I don't like what I'm sometimes modeling for them about marriage, because my husband and I aren't consistent in our parenting, and allow double standards for the kids in our home.
If I were in your position: not yet married; no kids together yet; and already aware of the parenting conflicts, maybe I would walk away. That's so hard to say, enmeshed as I am in the family I have. But I know for certain if my sister were in your position, I'd urge her to walk way. Love is not a panacea.