I started taking karate early this year as a way to get back in shape (45 yrs old) and learn a practical skill at the same time. I just couldn't make myself exercise without the possibility of gaining something useful.
I had several choices of discipline in this area and chose Okinawan Isshinryu for it's fairly low impact, get down to business style. Our Sensei studied under Master Wheeler and uses tapes of Master Shimabuku to refine our katas. I have found the practical self defense nature of Isshiryu to be very beneficial.
Since you have already studied Isshinryu you would not have to start at the bottom.