They were panning an interview 3 months in advance? That seems like a long lead.
My biggest advice when going into an interview is cliche, but true, be yourself. They are going to find out who you are anyway, so get there as quickly as you can. If it is a good fit, it is a good fit. If it sin’t, no amount of prep is going to change that.
My second piece of advice is to interview them as much as they are interviewing you. It seems like your dream job from the outside. Try to be objective to see if it meets your expectations.