Impressive mileage on the E90, especially given that it was the start of more tech under the hood and more expensive maintenance (electric water pump, no dipstick, etc.). As for long term reliability of the G20, that of course can't be known, however with cars becoming more and more rolling computers, it is expected that these cars will become very expensive to maintain to the 250k mile point of your old E90. I know in another thread a replacement laserlight headlight was $3k (without labor), so maybe an extended warranty is advised after some ownership depending on how much you trust the vehicle.
As for engines, I would stay away from the earlier 4-cylinders (N20 engine code), and go for something post 2016 with a 30i moniker, denoting the new B48 engine being used. Your best guess for long term reliability would be asking the F3X forum, as I know there are users who have 100k miles+ on their 3/4-series, but of course, beware, being that this is a forum so there may be some bias.
Hope that helped