first of all, welcome to the site!
i've been in a similar situation before. it won't be as daunting as it sounds. that's for sure.
that being said, here are some tips:
1. instead of developing an entire curriculum, pick a series of textbooks that come with a developed curriculum. then you'll have something that you can build on and add to. you dont have to do everything in the textbooks but at least it will get you started.
2. as far as having students coming and going at anytime with different levels, here's what i used to do at a previous school. this will work if you are the only teacher and your classes dont have to coinside with other teachers' schedules and curriculums. first start off with, for example, a level 1,2 & 3 class. those classes will probably progress at different rates. as the level 1 students move up (into the next textbook) and level 3 students move and start to move on (leave the school) you can just add another level 1 class. eventually, what you'll have are many classes at varying levels but you'll have a nice selection of levels. then when a new student enrolls, they take a level test and go to the appropriate class. that way there will probably always be at least one appropriate class for each student. the main thing you want to avoid is having really mixed classes, as im sure you already know.
3. if you can't have such a free schedule like i just mentioned, you might want to set it up so one book takes about 6 months or a year to get through (depending on the book and how often you see the kids). so, for example, every 6 months the students finish a book and move up a level.
the main challenges of the previous 2 systems is always having a beginner level class available.
now that i re-read your question im not sure if im even answering your question...
anyway, pick a book (preferrable one with a series containing 4-6 different levels) and have your students progress through those. if you keep all the classes in the same series of books the students will get used to the structure of the class and it will be easier for you to make your lesson plans and eventually you'll be able to recycle your lessons as you start to re-teach the same book.
i recommend Let's Go, or for something more conversive, English Time. they both have teachers books, workbooks, student books, videos, tapes, flashcards, etc...
i hope you get something out of that ramblimg...
main point: don't worry! with your experience it won't be as overwhelming as it seems to be now.