The lowest carb ones are the creamy ones: Ranch, Caesar. The VERY lowest salad dressing is just oil and vinegar.

Dextrose is another word for a kind of sugar. I know of "Canola oil" (formerly called rapeseed) but just discovered that it's called Conola in the US in some places. Was is a syrup or an oil?

Usually in bottled dressings, if some kind of sugar is the 4th or 5th ingredient (or later) in the list, then it's pretty safe to use.

Check the "carbs per serving" - if it's about 2-3 in 2 tablespoons, then I'd go ahead and enjoy it. You can cut the carbs by tossing the salad with some olive oil first. (This is called 'fatiguing the lettuce'). Then you add some of the bottled dressing (measure how much) and toss again.

If you're going to buy the ready made salads, how about something different than iceberg? Romaine and other dark lettuces have more nutrients than iceberg. Those 'baby greens' are good too - if more expensive.