I applaud you trying to think through how MBTI factors might play out in your relationship. And, there are a lot of people... especially NFs... on this forum that find it offensive to say that personality type precludes any relationship... And they are right. Two people of any two personality types can have a relationship. And, if they are mature and willing to work at it, the relationship can be generally conflict free.
Now, my Te is going to kick in while I tell you that the above statements are a load of horse shit....
Two people of any personality type can have a relationship... but that doesn't mean that they'll have a good relationship. Conflict free... after tons of work and tons of compromise... does NOT make a good relationship. I think this offends some people's belief that love will conquer all. And, a lot of NFs believe this. In fact I believe it. But love winning the battle doesn't mean love wins the war.
What is happiness in a relationship? Well everyone has to figure it out for themselves, but in my opinion, there is always a logical give and take. A sense of reciprocity. A fundamental sense of fairness that what one gives brings a valuable and great return.
Say I were married to an INFJ who is by some theories supposed to be (along with an INTJ) my ideal complement. Well, I've dated INFJs. And, boy they can be the most obstinate, overly analytical bunch EVER. Oy vay!!!!!! But learning to live with this differing point-of-view is well worth it to me because they give me something I want in return... nay crave... namely a transcendent experience. INFJs and I are on the same wavelength. So alike in how we look at the world in some ways. So different in others. But when we connect it's like magic. (And same with INTJs by the way.)
Now take the ISTJ and ENFP relationship. Can it work for some people. Well, I never say never. But IN GENERAL I would say that the pay off for an ENFP in such a relationship is rather predictable. The ENFP would get safety and stability. The ISTJ would get pizazz and romance. But here's the problem:
In GENERAL, an ENFP that picks stability in such a relationship is not living up to their full ENFP-ness. The very core, the very essence of being an ENFP seems to be finding meaning and meaningful relationships in everything she/he does. For an ISTJ, this is not what makes them tick. In fact, their worldview in some ways extinguishes this perspective. Thus in GENERAL, I would say this is a bad, though not impossible, match.
Life is so short and so hard. Why not date someone who doesn't require so much work to find common ground? If I dated an ISTJ, I would find his stability very appealing. But eventually, I'd want to be my true self. I'd want to have long, passionate discussions, and meaningful connections with people, and have tons of analytical friends. My ISTJ would not be able to go there with me. AND THERE IS NOTHING WRONG OR SELFISH ABOUT ME BEING HONEST WITH MYSELF ABOUT MY NEEDS IN THESE AREAS.
Could I make a relationship with an ISTJ work? Yeah, probably. But I choose not to. I choose to spend my energies, not on compromising about all the things I'd have to compromise with my hypothetical ISTJ boyfriend. I think their are a lot of ENFPs out there that find such putting one's own needs first as selfish and morally wrong. In fact, ENFPs are so good at tailoring their approach and putting other's needs in front of their own that they can wind up in relationships that don't really work for them in the long haul. I think that for ENFPs in particular, knowing who you are and knowing what you want (rather than tailoring one's approach to the people around you) is the key to self-actualization.
So I encourage you to have fun, but be honest with your needs, too. Do you want to spend the next 50 years of your life with someone who in some ways is speaking a different language than you (S/N)... that you will forever have to translate? 'Cause never doubt that meaningful, long-term relationships is the real goal of most ENFPs. So if you decide to seriously date this girl, you owe it to her... and yourself... to be honest about what about her will bug you in 10 years. This isn't being pessimistic. This is being realistic... and mature in your pursuit of relationship happiness.