I doubt there really is a 100% fitting word, but noveau riche or young money would fit a lot better than this I'd agree.

土豪 is slang and a relatively new term, also a derogatory one. "土" here denotes a similar meaning and feel to "country bumpkin" or "coarse/uncouth/unrefined" in English, while "豪" references extravagance. Put together, it has a similar connotation as noveau-riche. Duolingo using "stuff" to translate "东西" helps to convey the casual and somewhat-critical tone of the original Chinese.

My guess is the two-character rule. 贵客 and 贵宾 are words on their own, but otherwise you would normally need 的 between the adjective and noun.

Accepted now.