I would consider importing a loaded Signature model from Aus being right hand drive.
I still disagree on your other points.
A small percentage of every that car made that has a chain and for example a direct injection system will suffer from similar problems. IIRC approximately 230,000 Mazda6 units were sold in Europe between 2008 and 2013, where the largest share were diesel. I can't recall how many Mazda 6 2.2 diesels from that era are still "on the road", but if the problems that are discussed on this and other forums are as widespread and disastrous as some people would like you to believe, there wouldn't be enough garages in the UK to keep these things on the road.
Cars are designed to be much more economical and pollution friendly than they were in the 80's when I first started working on them. The demand on some components to operate within very right tolerances is supplemented by engine management units that will compensate in real-time to what i happening in the engine, and small issues can be rectified. There are many issues that afflict most cars e.g. DI technology (since the early 2000s) that by their very nature of injecting fuel into the combustion chamber increase the risk of two common problems.
1) Carbon build up on valves, valve stems and valve seats
2) Leaky injector seals
On top of that, you're subjected to the usual array of issues with clogged up EGRs, throttle bodies, and PCV, MAP/MAF issues to name a few. The now more common formula of turbos and EGRs causes that build up of sooty carbon deposits by virtue of the way in which they are designed to re-circulate the hot oily gases.
Mazda like any other manufacturer buys injectors from experts such as Bosch or Denso, they don't make their own hence why you will see same symptoms in other manufacturers cars using similar technology.
Some of the problems that can occur with engines are down to the way in which the car is being used, e.g. short stop start journeys vs long motorway runs, and by this, I do not mean that the driver/owner is neglecting their vehicle, but that the usage profile or operating characteristics influence the way in which the engine is affected. For example, I have seen evidence where DI engine cars that tend to do those daily short and stop start journeys suffer with carbon build up in areas that can't be treated by engine oil or fuel additives and even at 20k miles they are beginning to sound and run rough, MPG figures drop, and without appropriate maintenance/treatment more severe problems occur.
At the same time that technology has changed in vehicles, a lot of garages/mechanics are becoming pressured about how much work they can do in less time, and as such spend little or no time trying to diagnose faults fully - and that's assuming they can even diagnose faults. Far too many garages are staffed by those who will only refer to a fault code reader and can only replace parts. Mazda are no different to other dealers and some of the dealerships will suffer the same issues i describe. That's why I prefer to diagnose problems myself.
To address some of the carbon build up related issues I mentioned, BMW recommend walnut cleaning shell cleaning/blasting of inlet valves and ports, in addition to the oil/fuel treatment and maintenance procedures for other engine components. It's nothing new it's just all part of what I would call routine maintenance.
If the extent of problems with chain stretch were so severe, Mazda would have invested more to re-engineer the chain and put a recall note out. I asked Mazda UK shortly after I bought my SL about the chain stretch issue and they have never revealed what would trigger a recall. However, in their view the problems were small enough to not warrant taking that kind of action considering the tens of thousands of chain-equipped GH diesels they sold. I admit, like a lot of companies, Mazda's PR should have been better about the chain stretch issue even if it did only affect a small number of users, as no one likes to shell out £700 - £1200 for a chain replacement job.
I don't mind paying Mazda £220 - £270 a year to service my car in respect of their "expert knowledge" of their own engine and fuel system. It gives me some assurance about the chain, on top of anything I can see from my ODBII reader. The rest I can take care of myself.
Given what I knew about the chain issue, I still took that risk to buy my 2009 SL. I weighed up all of the other cars I could afford against the list of owner issues and the likelihood of them happening to me in addition to the specification of car I could get for my money. The SL won hands down and I've been rewarded with 3years to trouble-free motoring in a used car. If I can accomplish a sufficient amount of sound-deadening in the next two months, I will probably keep this car for another 3 years and including changing the chain when the time i right. Hopefully, it won't snap on me unexpectedly. If I keep it that long and get a new chain, the cost of buying and owning that car will be less than £1300 per year over 6years, which includes all weather tyres, new stereo, brakes, and servicing. For a Yorkshireman, that's pretty good value and better than paying £4200 - £4500 PCP per year for a new top of the range 2018 model.
I would also recommend that if you are about to buy a 6, take along your ODBII reader and plug it in before going on a test drive. No seller will stop you from reading codes if they have nothing to hide. If you're still not sure, take a mechanic along who knows what he's doing to give it a once over. I bought my car privately and I spent a least 30minutes looking under and over it, a 30minute test drive which involved me in the passenger seat first, followed by me driving it in a variety of conditions to check as man of the aspects of the running gear as I could. I even drove it with the windows down to listen for odd noises.
The only time you can't check a car thoroughly before you buy is at a car auction. I've just spent a year working closely with a global car auction business so I'm familiar with how they work.