Personally I've never heard of any significant issues with injectors / hpfp's on the newer n55 35i engine. The older twin turbo n54 which I don't believe was ever offered in the e70 had some reliability issues. A lot of folks confuse the N54/N55 engines which are not the same beast. In fact I would call the N55 one of BMWs more reliable engines of the recent past. Some carbon build up is an issue on any modern direct injected engine, but frankly for most won't rear its head very often and is pretty cheap to clean.

Personally if reliability is your goal, I would go with the newest E70 you can find preferably with 35i engine. Also I'd go with a newer car that didn't have the slow as molasses CCC navigation system which also dosen't have any map updates after 2015.

As for the diesels... Their emissions reliability is bad but has gotten better in recent years. My diesel I have tuned and deleted the EGR which is the biggest source of issues, but still have the DPF and SCR in place.

Personally I like the V8 and had a E70 n62 4.8i and now drive a S63 M. Both are good engines, I think give how overweight the e70 is that the non-turbo 6 would be a slug in this car. Especially in two lane passing maneuvers or towing. I upgraded from the 4.8i to the M as I found the 4.8i to not be relaxed while towing. Basically I had to keep it in 4th-5th gear (6spd auto), to keep it at 70mph with it floored on anything but a flat road. The turbocharged engines tow much more effortlessly and in 6th gear with better fuel economy. Yes, my M does significantly better in fuel economy than my 4.8i did while towing