my main advice is to have the patience of a Saint, the fortitude of a Marine, and a sense of humor like no other.
Only two of my students are in the category of Multiple Disabilities and I've only been doing this for four years myself but it doesn't change my advice.
I would ask about the caseload numbers--how many students overall and how many of each category--and the amount of other adult assistance available--be it co-teachers, paras, nurses, etc. It is common sense that the worse the disabilities, the lower the student count should be...
Also ask about your specific responsibilites for health and hygiene issues--diapers, toileting, brushing teeth, mobility and transportation issues, medications, feeding tubes, and so on. The classes themselves may be based on low level academics, social skills or life skills. I am very frustrated at times because I have these two who need endless attention to life and social skills yet still have to do very basic academics with my very low functioning MR/LD students. It is impossible to juggle at times.
Good luck though--these kids often have a lot of great qualities that are sometimes easy to overlook in light of all of their obvious problems.