The sensei has modified the 'official' katas of the style. She 'mixed' some parts of pinan shodan, with pinan nidan and so.
That's pretty worrying. It's not uncommon for schools to have slightly different "interpretations" of the same gross movement (e.g. one to say something's a block while another says it's a strike - but the limbs are moving in roughly the same way). It's another thing to arbitrarily change entire sections of kata. I've more tolerance for instructors supplementing kata with one or two unique to the school, though have never been very comfortable with that either. Still, in the end this won't have a particularly significant effect on how well you learn the art, so I'd say it's more a troubling sign than a actual problem. If you get the chance, you might ask her about the reasons for the changes - if she bites your head off, or is vague on justifications - both signs it's better not to train there.
I have heard the sensei bad mouthing other senseis of the same style. Seems she does not like the local community of the style. I asked her about going to some weekend short courses organized by the community and she said 'I can not forbid you to go if you want', but it is more up to individual initiative and the club does not participate.
There are all sorts of possible reasons for that, some justified and some not. For example, she might have found in the past that these other sensei presumed to "correct" or instruct her students with what she's convinced is poor advice; deeming the changed kata "wrong" would be one likely area of contention. There could also be tensions left over from previous discussions about merging into some larger organisation, or collaborative events in which there were misunderstandings about financing or roles etc.. If you do train there, perhaps keep an ear out for the exact things she says about other senseis - see what motivates them. It's a good sign that her students are friendly - suggests she's not just a spiteful person in general, creating a toxic training environment. It's also a good sign that she's part of the national martial arts association - at least she's prepared and able to interact with some other instructors and schools.
Is somewhat expensive. Starting september the sensei wants a year paid in advance. Is a lot of money and I told her, 'I can pay half a year' and she said 'I have to consult with my business partner, we can see how to fix it', but still the cost is somewhat over the average.
That's a bad sign, considerably worse than asking for direct debit payments which would also give her reasonable financial security, but has to be weighed against the skill, experience, knowledge and reputation she's built up. If her tuition is clearly in demand, and you see the existing students are of a high standard, with a good number at all grades, and they clearly respect her, then it may be worthwhile. Still, if you can pay the next few months - prior to September - without committing further just yet, it gives you enough time to decide whether to renew for a year.
On balance, I'd say give it a try if you can afford it.