Is it ethical to get suggestions/corrections about my PhD work from a consultant?
You need to do two things here:


You should ask your advisor in advance whether this is acceptable.  The answer may depend on the details of your situation (exactly what is involved in this CAD verification and how central it is to your research), and your advisor is one of the few people who can give you a definitive answer.  I'd bet that it will be OK as long as you do the design yourself, but my opinion is not so relevant: if you end up in a disagreement with your advisor, it won't help if you complain that people on the internet assured you it would be OK.
You must acknowledge this assistance in your dissertation and papers, the same as any other contribution to the work.  This is a matter of intellectual honesty.  However, acknowledging the assistance afterwards can't substitute for asking your advisor in advance.  (In particular, your advisor might say "There's no issue of honesty, since you described exactly what you did.  However, you hired someone to complete a substantial part of your thesis, so you have not done enough yourself to graduate.")