Due diligence of code - crucial when building, buying or selling a business

Hi there
How do you handle the Due diligence of source-code/cloud-setup when buying/selling or building a business?
I found that a lot is skipping this step and later find out that they a left with a bad none scalable setup that will cost them thousands and thousands of dollars.
So how do you handle it?
Asking because I am thinking about building a little business dedicated to do Due diligence - please share your thoughts.
Thank you.