Learn Swedish: Vad är det för knopp? - What kind of bud is that?

This split is the standard way to ask "what kind of" in Swedish. Instead of keeping the phrase together at the beginning, Swedish starts with the question word vad (what), puts the verb är (is) and subject det (that) in the second and third positions, and drops för right before the noun knopp (bud).

Could I use vilken instead of vad ... för here?

They actually mean different things! You would use vilken knopp if you wanted to ask "which bud?" to identify a specific one from a group. Because we are asking "what kind of bud?" to find out its species or characteristics, we must use the vad ... för construction.

Can I say vad är det för en knopp instead?

Yes, absolutely. It is very common in spoken Swedish to include the article and say vad är det för en knopp (or ett for ett-words). Dropping the article, as done in this sentence, is also perfectly correct and often sounds just a little bit more concise.

How would the word order change if I made this an indirect question, like "I wonder what kind of bud that is"?