And nobody has mentioned driving licence catagory's yet
From experience I would go for the Twin Axle setup, If not only for the Weight carrying , Then for the extra set of brakes , especially if towing behind the Mondeo. I hate being pushed by a trailer.
Just be carefull as you might be getting close to your towing Limit.(Remember to factor in all the Fuel carried onboard @ approx 730g / Litre)
A 4 wheel trailer will also be a lot more difficult to move around.
I dont think trailer guy was trying to sell you a trailer , just advise "Make sure the rest of the trailer is upto the job" before you invest in your current trailer , Ben has given lots of usefull advice on here with no Numeric Reward, and is a Valued member of the Ribnet Forum.
If you post a pic or 2 maybe people would have more of an Idea what you already have to work with, and advise better.
Out of curiosity was it the Blue rib on ebay a couple weeks ago up the yorkshire area ?,If so Is it a Humber ??