I'm in the process of submitting my I-485 and know I must declare this arrest but will it prevent the granting of a green card?

You need to declare and present all evidence of the disposition of the case. It should not affect your getting the green card but if you do not disclose it, it will show up anyway when you are fingerprinted and then you will be asked to present proof of what happened and you are signing the petition under penalty of perjury.

You are correct about the disclosure: you have to tell USCIS about all your arrests. USCIS runs criminal record checks on every 485 applicant, and your arrest will come up on your FBI rap sheet even if the arrest record was sealed or expunged on the state level. However, an arrest, by itself, does not disqualify you from adjustment of status, no matter what the arrest charges were. Only a conviction counts. Without a conviction, the only way an arrest can matter is if you fail to disclose it to the Service. Since the charges were dropped, list the arrest in your application, attach an notarized affidavit about what happened, a certificate of good conduct from the local police precinct or sheriff's office, a certified letter from the state attorney's office confirming that the arrest charges were dropped and were not brought in court, and a certified letter from the local criminal court that you have no convictions and no record of prosecuted offenses. The form and the wording of these documents wary from state to state (and even from one county to another), but, basically, USCIS will want to see confirmations of your story in your sworn statement, and a document from each, the local police, the local prosecutor, and the local criminal court. It is your duty to prove that you are eligible to adjust status; if you do not provide the listed documents, USCIS will send you a request for evidence and it will extend the time of processing of your case by several months.

You will ultimately find that you will need a competent lawyer in this situation. This is why. The burden of proof of eligibility for an immigration benefit is on the alien making that application, not the DHS. In this instance, your I-485 biometrics sent to FBI will show CIS of your arrest in FL. CIS is going to ask you for a certificate of disposition of that arrest As reasonable as it may have looked when you sought to expunge this record, it was, for your future immigration purposes not a wise step. Because, if your your criminal file was destroyed pursuant to your request for expungement, you may not have anything to provide CIS about the outcome of your arrest and just saying your record was expunged is not good enough for CIS since an expungement is not legally sufficient evidence proof of the outcome of your criminal case. So, do you think you need a lawyer?

So long as you disclose you should be okay. Dismissed criminal charges generally have little bearing on immigration cases.

While you must declare the arrest & provide certified copy of police report, a dismissal or non-conviction will not prevent you from obtaining your green card; but make sure you also provide certified copy of the court record.

The arrest by itself will not harm your case unless you were also charged and convicted of the crime. You will probably need to get a certified copy of the arrest report and a letter from the court stating that there is no record of any case against you.

Only conviction affect the green card process, not arrests. If you never convicted of a crime, you should be fine. I suggest that you go to the court where your case was originally filed and obtain a copy of the disposition showing that there was no conviction.