Here is where the old saying becomes a golden rule: Less is more.
BMW built its reputation by creating cars that were already exceptional from the factory. Balanced chassis, responsive engines, rear wheel drive and a connection between driver and machine became part of what made generations of BMWs special. Stock Class celebrates what BMW engineered in the first place.
You don't need to become a race car engineer before becoming a race car driver. Maybe your BMW is sitting in the backyard. Maybe it's been forgotten behind the shop with four flat tires. Maybe you found it on Marketplace for less than somebody else's set of wheels. Perfect. Get it running, freshen it up, make it mechanically healthy and install the required racing safety equipment.
Then follow perhaps the simplest endurance racing strategy of all: