We have a few software processes in place and they are ISO compliant. Our company now wants to go for CMMi level3. Is it better that they first align their processes before involving a Lead Appraiser or CMMI consultant, or involve a Lead Appraiser or CMMI consultant now and let them help us?
Well, I guess that depends on how much re-work you want to complete :)
A good CMMI Consultant (who may also be a Lead Appraiser) can not only help you but can also point out a few hundred mistakes that you shouldn't make - making their cost almost zero when you consider the re-work involved.
The biggest problem is that people don't always understand the CMMI Model itself, so without a little guidance go off and create process monsters that will take years to fix.
Do yourself a favor and seek some professional help early - just make sure it's the right guy (or girl)!