Is or are?

Matthew WaiIs it correct to say "what is important is commodity prices and economic conditions"?

As you can see, it is awkward no matter how you arrange it, so re-cast:

Commodity prices and economic conditions are the important factors.

I know it is awkward but I want to find out whether it is correct to use "is". For example, will it be correct to say "what he said is reasonable" if he has said many things?

Matthew Waiwill it be correct to say "what he said is reasonable" if he has said many things?

Yes, that's fine. What he said is treated as singular, even though it may encompass many ideas.
In your original sentence you run into problems because you start out with the singular what is important... and then end with the plural

Similarly, is it correct to say "what I want is two apples and two bananas"?

Matthew WaiSimilarly, is it correct to say "what I want is two apples and two bananas"?

In this case are is preferable, although you might well find native speakers using is.

What he said is reasonable.
What he said are reasonable ideas.

Are they both correct?

The first is fine.
The second doesn't work. "What he said is reasonable ideas" doesn't work either. You would have to rephrase this.

"His ideas are reasonable."
"He has said reasonable ideas."
Do they work?

His ideas are reasonable. Fine.
He has said reasonable ideas. No. That doesn't work. You could say He has offered/suggested/proposed reasonable ideas.

Matthew WaiHe has said reasonable ideas

'Said' requires either a quotation or a clause:

He has said, "I had some reasonable ideas."
He has said that he had some reasonable ideas.