If the DA says to plead guilty with a letter of explanation down to 83 mph then I would go with that. Your only other option is to plead not guilty which then in turn requires you to go back to court in person to plead your case in front of the DA and the judge.
Since you were doing 10 mph+ over the posted speed limit, your fine, including court fees will probably be around $250 or so. Also, normally you would have points put on your license that would affect your insurance, however, I do not know how that would work with you having an out of country license. Maybe the DA told you something regarding that?
If you have the money, and don't have to worry about points on your license, I would just plead guilty and pay the fine if I were you.
Here is a link to the dept of motor vehicles for NYS.
NYS DMV - Frequently Asked Questions
Hope this helps some......