Since you are from Ohio I'll say a 3mm should be OK. Bring a hood if you get cool you can wear it. I've also got a 1mm vest I slip under my 3mm when the water temps drop below 70.
I can only echo what others have said here, Key Largo to Marathon has the best diving. Key West has the best partying. I really like Capt Gary at Conch republic in Tavernier. If you need nearby accomodations his wife can help find you a place.
I like Bluewater & HMS Minnow further north in Key Largo but's there's a few more good outfits up there as well. Been a while since I dove out of Marathon so I can't help out there.
The Eagle, Benwood, Thunderbolt, Bibb, Duane, and of course the Speigle Grove are all fun wreck dives although some of them can be challenging at times. I like Molasses and French reefs off of Key Largo, see if Conch Republic is doing a drift dive on Conch Reef that's a blast.
AL