As you mentioned it has a whining turbo, an indication of poor lubrication or excessive boost wearing the bearings.

It's up to you and your customer how far you go and lengths to cover your arse and warranty issues.

Service history might help, has it had regular oil changes?

These engines are notorious for turbo failure, I put it down to poor servicing.
I've never replaced an oil pump, I think that's a bit over the top, but I have seen many cars with a blocked strainer, so it's a consideration.
I normally just give the engine an extended engine flush, flush and clean the turbo oil feed pipe and then use a good quality oil and filter, never been caught out so far, touching wood with fingers crossed

Others may advise differently of course.