Why did Hunter take the job?

The money, for one reason.

But against the money was Biden and Obama trying to get corruption investigations going. The IMF wanted investigations as did most of the EU, evidently. Yet Hunter takes a job with potentially one of the most corrupt.

Why?
One answer sticks out besides money. Hunter was hired to help clean the place up as the company wanted to sell gas to Europe and corruption charges were hindering its effort.  Without evidence, and assuming Hunter Biden had any use at all, this would be my guess.

Was Hunter using his dad's influence? Yes, under this theory, he was pressuring his dad to do more complaining about corruption, Ukrainian corruption was hindering the company.

This is the common sense rational default hypothesis; assume the parties were on the up and up.  But I doubt the rational, and this is our problem.  The Swamp dealings are never expected to be rational or  up and up.  I have the opposite reaction, I am surprised when I cannot find corruption. I still haven't found any corruption with Chuck Schumer, for example., but I am certain it is there.

I read a little about Hunter, he seems OK, but not nearly smart enough to execute such a rational story. If I got that job because of a famous relative, you can bet I would be sitting on my butt, doing nothing.