Hello PossibleFuturwSM, welcome to the group

You found a good place to think through what you want in a relationship and in your life. Many stepmoms here can share their thoughts and experiences with you, to help you decide. I'm one of those stepmoms; my H and I have been married almost 10 years and together for almost 12. The kids were just-turned-6 and almost-4 when we got together, and are now 14 & almost-17. So, it's been quite a while!
My H's kids' mom has many BPD type traits -- blaming, not taking responsibility, rehashing the past, badmouthing him to the kids, allegations of "emotional abuse", twisting words/what was said, etc. She never escalated to police, though in a way, I almost wish she had -- just to have someone else see how not in touch with reality she could be. After the divorce, she quickly married my H's former best friend, who has many NPD type traits. That dynamic (BPD + NPD traits and behaviors) has been very hard for us to stand up to.
At the time of the divorce, H thought that even though he and the kids' mom couldn't be married successfully, of course that wouldn't translate to coparenting, and he believed they could set aside differences for the kids. So the original "parenting plan" just said "H can be with the kids whenever he's not at work". He let Mom have full custody (in our state it isn't separated between physical/legal), again believing it didn't really matter on paper, because she'd be able to coparent fairly.
It has been very challenging since then, as her sense of entitlement, and inability to see the kids as different from herself, and her desire to make H pay, have intertwined with her uNPDh's desire to replace H in the kids' lives.
We have had to work extra hard to build some non-intuitive parenting and communication skills, to counter the narratives coming from Mom's house. It is tricky to find times to just "relax and be me" with the kids, as I'm often on high alert listening for "Mom's drama" behind the kids' words, and feeling a need to be ready to be validating, nonreactive, supportive, and an active listener.
I would not call it easy to stepparent when the kids' other parent has PD type behaviors. It has definitely taken a toll on the relationship between me and H. We have a marriage counselor we've seen for almost 10 years, and I added an individual counselor over a year ago. We are still working through some difficult stuff. It can be legitimately traumatizing to stepparent.
That's the nutshell version

! All that being said, we have really good times, too; it's been a gift to be able to support the kids in their interests and watch them grow. We have made our own traditions and discovered new cool things we all love to do together. There is sometimes a kind of freedom that comes as a stepparent, where you can let go of the "disciplinarian" role and instead focus on listening, validation, and just keeping the kids talking.
So, with that out of the way...
how long have you and he been together?
have you met the kids yet?
what's the custody situation right now?
does your H have a lawyer?
where does everyone live? same town? far apart?
...
Some stress in the relationship can come from nebulous custody/parenting time issues. It may be a more manageable relationship if custody/PT is very locked down and very specific, so that there are real, enforceable boundaries/consequences on Mom. If things are more loosey-goosey, you may be in for many years of stress, ambiguity, and financial challenges.
So, to answer your question
if you were in a similar situation and had this information this soon into a relationship, would you end the relationship with the great guy?
hmmm... looking back on me in my 20s, when I met my H, yes I knew he was divorced, but no I didn't know about the high conflict, and even as it unrolled, it was still confusing. I didn't have this site right at the start -- but you do now. And, you know how it is, you meet someone, there's chemistry

... for me, it was so easy not to give weight to the dysfunctional stuff. I was in love and I didn't know about BPD! To my credit in the past, though, I did know that dating a guy with kids was no joke and nothing to mess around with -- you're in or out, for the kids' sake. So if I had info about BPD and the BPD/NPD dynamic, and what was in store... it would have been harder for me to commit, and I may have more seriously considered calling things off before meeting the kids.
As you have an adult child, I'm guessing you're older now than I was then, and you're looking to make some wise choices before getting more serious. Smart move. This is a really good place to work through those options. Write back whenever works for you -- I'm looking forward to hearing more about your situation.
-kells76