First off, size charts are not the end all and be all but they are a piece of the puzzle.
Have you had a drysuit course are are you working with a mentor? Even seasoned divers can find themselves diving like turds for 10 or more dives while they iron out their configuration and get their head into the skills. How good was your cold water wetsuit buoyancy to begin with? I'm not throwing stones but there's not much in your post or profile.
A weight belt might help? If you are wearing enough to be comfortable in Newfoundland without a belt (all integrated?) then you configuration does need some work.
I suggest (regardless of your ultimate drysuit) that you get a weight harness to let you better control your weight and trim while balancing your rig. if nothing else that BC must be miserably heavy slug of a thing right now. The DUI Weight & Trim is very common. The sizing should be irrelevant to the weighting. Extra material will collapse and not represent a need for lead.
Extra room around the Torso? That's relative but remember that you need room to bend and stretch. Assuming this is a laminate (bag) suit there is no stretch.Some loose material is needed to allow contorting like putting fins on and reaching valves. It will not fit like a Tuxedo. IMO most sizing heartburn comes from length issues.
It's a used suit, I'd flip it before getting it tailored.
If you bought it used from a shop I would push back for refund, credit etcetera if you really feel they "sold you the rack". At worst, they have the same used suit on the rack. If they deserve your business then they should be able to dispel your concerns with training/configuration. Keep in mind that aside from the suit, garments and other gear adjustments, especially weighting and fins often need to be added. If your BC was a close fit wet it may even be inadequate for diving in a warm drysuit.
Pete
