1. The latest BMW (car), which is equipped with many high-tech gadgets, costs at least fifty thousand dollars.
(It's normal to just say the brand of the car, if it's clear that cars are the theme. It's much more common to use numbers for sums of money e.g. $50,000)
2. It's correct if you include commas around the "which" clauses and the "the" you mentioned. But the style is difficult for readers because there is too much information between the first subject ("the closet") and the main verb ("were used"). I recommend breaking it into a sentence which begins: "Two things which were used by a 16th century British king are ...."
The British word for "closet" is "wardrobe".
3. "double-decker" is the word in the UK. I don't know if "double-decked" is used elsewhere. The word is a good one but it may not be commonly used. Merriam Webster (US dictionary) gives "double-deck".