Selling a 330i and 2005 WRX probably doesn't net you enough money to get into an E60 M5 with a warranty. So it all depends on what you want and how much money you're willing to put in on top. That car is obviously on another level compared to an E46 or a blobeye WRX. But, like you said...it wasn't built to be worked on in your garage for many things.
Do you want a high powered, complex, electrically sophisticated, monster of a full sized sedan?