Translation:My friend wants to buy that piece of clothing.
Here in the US, no one says "article of clothing." The phrase "piece of clothing" is a little odd, but still sounds slightly better than what you suggested.
In everyday speech, when talking about one item of clothing, we usually just say its actual name. "My friend wants to buy that belt / that shirt / those jeans / etc."
"Item" and "item of clothing" are also acceptable and sound way more natural. The word "garment" is also correct, but it is not used by ordinary retail customers. It is mostly used by people who work in clothing stores, the fashion industry, or dry cleaning to describe merchandise.
I'm American. Piece is just weird. Article is archaic but ok. Item sounds slightly better than either of those, but still clumsy. Garment is super formal but probably my favorite.
The reality is, "clothing" doesn't translate well to/from Chinese. We'd use specific words ("shirts" "dresses") or say "items" or "things" without explaining that they're clothes. That difference between the languages means that learning this word will be accompanied by unwieldy and weird-sounding English.
I agree your translation is fine. " which should be 要。" ...I don't agree with the "should" part of your statement. These words are very flexible and 要 has several meanings, one of which is want. There are other words that can be used for want. So there is a lot of overlap.
You could probably use just 想 here but 想要 is quite common and useful to know. I suspect people use it in this case because 我想買 is three consecutive third tones. This one https://www.chineseboost.com/grammar/xiang3-yao4-xiang3yao4-difference/ discusses the three combinations though doesn't really answer your question.
See also the post by Wulfgeri in this thread, and this link: https://www.chineseboost.com/grammar/xiang3-yao4-xiang3yao4-difference/