Hi
The 3.0d you have should be 218 bhp with an decent ECU tune you should see 250 bhp and with a few more tweaks 300 bhp is in reach and the motor stays reliable, but it's the increase in torque that is most noticeable especially in the midrange.
Your Gearbox will be ZF GA6HP26Z BMW like a few other manufacturers started the "sealed for life" myth what they mean is sealed for their 3 year Warranty period 🤣 ZF themselves recommend Fluid and Filer changes at 80 to 100k/km so about 60k miles. BMW got a bit Red Faced and never ones to miss a profit sell the fluid now. The boxes are pretty tough the transfer boxes can be damaged quite quickly by wrong sized tyres causing Transmission wind up. So when you stop if you hear a clicking sound or if you reverse slowly on full lock and get a jerking that's wind up so check the tyres. If the transfer box is damaged it is a DIY fix and fairly straight forward.
The best would be get a copy of ISTA/INPA and put it on a laptop (I have an old Win8 laptop in the Garage with BMW 1.4.0, INPA/ISTA, Mercedes Star and Volvo diagnostics on it plus a couple of others to help out the Kids and Neighbours. Once you get it up and running you will have Dealer level access I have seen complete laptops ISTA mounted on Ebay for £150 which is less than a Dealer would charge for a Diagnostic session. There are also others like Foxwell and iCarsoft
Check your battery often the cause of Gremlins BMW are power hungry beasts if voltage drops it will start throwing warnings. Also BMW like common earth points make sure they are clean so a session with contact cleaner may help.
Dave