Aloha, as a member of the Honolulu Symphony, experienced music teacher and president of the Hawaii American String Teacher's Association, I think that you will have a great time learning to play the viola. Since you have played several instruments before, I think you will find that you can progress quite rapidly.
Choosing the right teacher is the most important factor in helping you quickly reach your goal of playing in a community orchestra. I would avoid a teacher who specializes in starting very you children (2-3 year olds) and produces "prodigy" type students. These teachers are very comfortable spending a lot of time -- sometimes years -- perfecting each step of the process. This is fine if the students is 3 years old and time is plentiful.
I suggest finding a teacher in your area who has spent time teaching beginning strings in a public school setting. This person typically starts kids who are in the 5th or 6th grade and is accustomed to having students read and perform in an orchestra setting right away. This sort of person will focus on the specific "orchestra skills" that you will need.
I hope this helps!!
With Warmest Aloha,
Dan Padilla
President, Hawaii String Teachers' Association
http://www.examiner.com/x-7007-Honolulu-Music-Examiner