A withhold of adjudication will not affect your rights because you were not adjudicated guilty. As long as you have completed your probation, you don’t need to worry about being adjudicated guilty for this crime.
As far as getting it off of your record, this will depend on what you were charged with. You can seek to seal the records, or in some cases, expunge the records, but there is a somewhat lengthy list of crimes that will not be sealed or adjudicated. If your crime was sexual or violent, you may not be able to get it sealed. To view the list of prohibited crimes, go to the FDLE website for sealing/expunging records. You can however, purchase the firearm that you were asking about. Keep in mind that Florida courts will typically give you one withhold of adjudication. On the second and subsequent charges, they will not offer a withhold, so stay out of trouble.