Welcome to the Goldwing family (soon to be anyways). A lot of good advice on this thread already.
With your size and riding experience, I do not think you will have a bit of problem transitioning into the 1500 or 1800.
My opinion on the 93 and what seems to be a fair trade that has already come up for you.
Personally I would not make the deal. While it IS a good deal, you seem to have a little wiggle room in that you are not so desperate that it is all you will be able to afford. Remember it is a 20 year old bike already with 100K on the odometer. Granted that it will continue to be a good running bike for many miles to come. But why take the chance and jump on the first bike that comes up. Shop around.
Take a look at this one here, http://www.cycletrader.com/listing/1998-Honda-Goldwing-GL1500-108158764
I found that one within a few minutes of doing a search. It is close enough and would be a good first trip for you to fly out and ride home by yourself and learn the new bike. Plus it is low enough miles that you will be almost guaranteed another 100K miles out of it if you chose to stay with the 1500.
LOL you do have quite the decision to make here and lots of choices out there. Just my own opinion on things. (you know what they say about opinions and ******** :ssshh
Ohhh and make sure you take a look at the events thread and try to make some of the M&G's. Loads of great ppl on this board.
Keep us updated on what you decide on in the end. :waving: