What were the "mechanical issues"? HPFP, injectors, wastegates, turbos? I don't know about the whine of the turbos but my car definitely sounds different for the first few seconds after I start it up and then becomes quieter. With my recent mods it's even more noticable.
If you're a DIY BMWer...I wouldn't be too intimidated by these cars. A bit finicky yes, but if you're buying this car as an enthusiast and enjoy working on cars..turbos, HPFPs, injectors, coil packs....none of it will be that big of a deal. The turbos obviously a bit more imvolved as they're harder to get to but there are a lot of great DIYs that walk you through all of this.
When I bought my car I was a bit skeptical about it all but now that I've spent more time with it and do all the work myself...not a big deal. If my turbos go...upgraded turbos....no worries...actually a good excuse
So I'd say, if you're getting a good deal and the turbos are okay (hard to tell from an internet discription) and you don't mind or even like wrenching...go for it!
Edit: there are some videos
that have sound clips of startups when people were comparing exhausts...might want to look for a couple of those and listen to them to see if it sounds like the one you're considering.