Jeremy is being modest here. He may have saved you weeks of research, especially when you consider all of the inaccurate misinformation you would have to sort through to boil it down like this. Excellent writing!
I encourage you to take the time to read the links that Jeremy provided. Excellent in-depth stuff!
Below are some things that I'd add:
MAINTENANCE
At 50k miles you are at the perfect time to jump on maintenance items that will extend the life of your car and prevent some issues that will likely bite you in the next 5k-20k miles.
These would cost a fortune at a dealer or even if done one at a time. This is an investment guaranteed to save money if you plan to keep the car.
I found an excellent independent BMW mechanic on BimmerShops website. He charges me $120/hr when I bring him parts. I booked a day of his time and bought everything at FCP Euro. All BMW fluids unless there were obvious choices to get the same thing cheaper without the BMW label, like the ZF transmission service kit.
Here's my list:
Automatic transmission service- ZF Kit
Fluids for front and rear differentials, transfer case
Coolant service including replacing coolant return hose & Mickey Mouse hose. Upgrading MM from plastic to aluminum.
Kit to replace serpentine belt, tensioner, etc.
At same time add a Kies Seal Guard to prevent engine failure.
https://www.kiesmotorsports.com/prod...7&_ss=r?aff=22
CHOOSING BRANDS
As you research you will notice some guys say that a particular part is great and others say that it doesn't fit and caused them nightmares. That's because most of the parts are made in China and the quality control often sucks. Guys who chase cheap parts often complain when they get burned and it's really expensive to troubleshoot and repair.
I'd rather pay a little more for quality parts and enjoy driving my reliable car. Most of the aftermarket parts that have made their way onto my car through extensive research were made in the US, Germany or Europe. I've seen some quality stuff made in Taiwan.
BESIDES ENGINE TUNING
There is often too much focus on more and more engine tuning. For a daily driver, take the time to upgrade brakes, suspension, tires, audio, etc and your car will be more enjoyable to drive.
Just choosing better replacement parts when things wear out, can be a big enhancement. Things like:
Hawk 5.0 brake pads are a big improvement
Upgrading runflats to MPS4S
Upgrading tire size- such as 225/45-18 to 245/40-18
Stock struts/shocks to Koni Special Actives
SOME RECOMMENDATIONS
Drop-in free flow air filter from aFe or K&N will prove a small performance bump
Chargepipe- Evolution Racewerks makes a high quality CP in the USA. A high quality option is FTP.
https://www.kiesmotorsports.com/prod...2&_ss=r?aff=22
Tune- I've got BootMod3. A smooth powerful factory like tune with free updates, excellent tech support and an enthusiastic helpful user community. It comes with a free automatic transmission tune that I really like. It has an exhaust sound dashboard to control how you want the burbles and pops to sound.
I tell guys all of the time not to fixate on swapping downpipes etc. Just install a chargepipe, a fresh set of stock plugs and install a BootMod3 Stage1 tune. Then enjoy driving your car.
Intercooler- My favorite intercooler is the Wagner Competition EVO1. Very high quality, no heat soak and especially no turbo lag.
https://www.kiesmotorsports.com/prod...3&_ss=r?aff=22
An intercooler upgrade is one of those tweener mods. Technically it's optional if you have a Stage1 tune, but required if you have a slightly higher horsepower Stage2 tune. But there are other factors like if you really push the car at high revs, drive aggressively, track on weekends, live in a hot climate. Those will all be helped by an intercooler upgrade.
Another factor that can push towards an intercooler upgrade is that there is a labor overlap, so installing an intercooler at the same time as the chargepipe can save money.
Hope this helps!