I personaly built a bare board shop from the ground up.
This was 65K sq ft and I did put in a true clean room with
air locks, HEPA filters and the whole deal.  It proved to
be worth the time and effort as the yields were great
compared to shops which I worked in that did not have this
clean environment.  It is tough to do, and worth it if you
do not get close to 100% yield at the imaging point.
The effort is not worth it if you do not have problems
with these processes.