I don’t own this particular outfit but reading the reviews lots suggest sizing up one, so if it was me then I would go for a bigger size personally. I would assume it doesn’t come with any knickers as the description makes reference to “your favourite knickers”.
Hopefully someone who owns can confirm but thought I’d pop a comment in. I also find the size guides on the product page really helpful usually, and there’s a couple of references in the reviews that this is accurate so it’d be worth checking that out as well!
The reviews have a lot of opinions on the sizing. The most recent review has the persons measurements included that might help you but I find checking the size guide and the reviews very helpful.