Welcome @jharry - the price gap between your E91 and an F31 320d will pay for a lot of maintenance / fuel (would guess you'll get 6-7ish for your current car). The N52 in your car is a sound engine - esp once electric water pump has been replaced. You could replace the suspension on yours for $1500ish and will drive like a new one.
OTOH the F3x generation is probably a bit safer and the 320d will be very good on fuel (maybe 5.5-6L/100k combined running). The f3x Gran Turismo model looks gash (IMO) so I'd definitely keep your E91 if that were the alternative, the Gran Coupe on the other hand looks OK but possibly out of budget.