What if I'm not interested in getting into another relationship or partner, can I expect my gf to be a life partner or should I need to expand my horizon to avoid heartbreak in the future?
There is no guarantee against heartbreak, no matter if you're monogamous or poly or a swinger, bisexual or asexual or whatever.
Marriages can end in divorce, of course.
I'd say, you look deep within yourself and decide how far up the escalator you'd be satisfied with. Then you ask your gf how far up the escalator she wants to go, or can go. She'd have to consult with her husband. If you feel you really want to live with her, but her husband does not wish to share a home with you, then you have a problem and will have to seek a compromise, if possible. If you want to father a child by her, again, her husband has a say, and there are legal consequences as well.
If your sights are not set as high as that, there are other steps lower. Meeting her friends. Meeting her family. Taking vacations together. Spending birthdays or holidays together. All these are things to be negotiated.
For me? I've been with my partner 8 years, and I've had a couple other rather serious relationships (and several that didn't get into true love territory, but were more FWBish). I don't desire to live with anyone but my gf, but we are both fine with having a partner spend hours in our home. We are both fine with another partner having sex in our bedroom, or any room in the house. We are both content if one of our OSOs spends the night, to let them sleep together in the guest room. We are both fine with walking in on our partner having sex with her OSO in the kitchen or whatever. We are both fine with each other having an overnight or a weekend with an OSO. A longer trip than that could be on the table, but has never come up in reality.
Neither of us wants kids (I've got 3 adult kids and my gf is almost 40, so it's kind of late for her to start).
I am fine with one of my OSOs meeting any of my friends (since all my friends are open-minded, or queer, or kinky, with an understanding of how poly works). Some of my family members have met this or that OSO of mine, some haven't.
Your mileage may vary. There are as many ways to do poly as their are people doing it. The choices are yours, and your OSO's, to make.