Dually or single rear wheel 3500 purchase advice please!

While the tow capacity of a 3500 SRW is likely similar to a dually (if not slightly higher) the payload capacity is not. That large a trailer will put a lot of weight directly on the rear axle, that's really the issue, not towing capacity.
I think you would be much wiser to select a dually truck for that trailer; even if the numbers just barely work with a SRW.