If you want to get a conti car like 508, my advice get the diesel model. My mechanic customer got 1 508 diesel model with over 200k mileage on board. Very smooth and powerfull until today.

The only issue with peugeot is a lot of the plastic parts like to fail especially the power window gearing. My mechanic help the owner to purchase from oversea, i think from Taiwan after local spare part shop quoted very expensive just for few plastic gearing.

FYI, i knew a few owners still driving E60.
BMW service center does recommend them to change the transmission during 60k, not sure if your service center did not inform you about it or miss-inform.
The so call "lifetime" is just to let new owner fuzz free of no need to change fluid transmission during the warranty limit of 3 years or 60k. After that is your own already if the gearbox got any problem..

Depending on how the owner take care of it's ride. Like the diesel 508 owner i met before, he is happy with it's ride comfort, great fuel economy, high torque and easy maintenance of the diesel engine. Anyway he only pump Euro5 diesel fuel, that why no issue yet from the emission system (catalytic converter) even after 200k.

A lot of direct injection (Petrol or diesel) engine require good quality fuel to reduce the injector & emission system from failure and in the past if petrol version you need at least Ron97 above for Euro4 standard. But now i think no much issue for Ron95 as starting 2020 already Euro4 standard also.

For the carbon intake issue, all modern direct injection system engine will encounter. If want to be cheapskate can just buy valve cleaner and spray on the valves or intake. They are few videos doing this via youtube. My mechanic usually just open the intake manifold, spray with the cleaner and let it soak in few minutes before cleaning it and suck out the dirty stuff.

Yes, turbo engine tent to have more maintenance in term of seals, hose and gasket due to high heat & pressure it need to produce to get the high power. That why a lot of people hype about modern turbo engine, but once warranty out most mechanics will be very happy about it. If the owner like to drive like a maniac, turbo engine usually will not last long compare to normal N/A engine.

I forget to take a picture, last i met a E60 owner he does shown me the service booklet and it does shown the service recommendation for the 3rd year 60k mileage with transmission fluid and also with the filter change. If not mistaken from Auto Bavaria.