Thats a solid piece - 5mm thick, prolly 14" tall. Since the Dealers cup is 55 cm, its probably 35cm, standard Roor sizing. Definitely go with the 18mm joint. They should size the diffuser for you if you request it, the "to fit" option - otherwise its tough without having it in front of you to measure. It should also come with a basic bowl, but make sure to ask. Also, since you are buying the diffuser separately, it should come with a standard downstem too - again make sure to ask. Other than that WC is good from what I hear, plus they usually have a discount available - you can even call them to talk to a human and order/check stock

If you are looking for a U.S. Roor, lemme know...