Good day to you too and cheers to you deciding to go for a tune.
Let's tackle your questions which are totally legit to have for a car that is loved dearly.
I have a similar setup to what you are planning.
2021 540i Gen 2 B58 with TU pump and I am on stage 2 MHD, MHD flexfuel kit and XHP stage 2 tune for the transmission. One of the best enhancements to have. Lots of fun, pretty safe and best bang for your buck.
Your transmission tune stage and engine tune stage are independent.
on xhp website you can see all the features each stage provides and choose accordingly.
https://rbttuning.atlassian.net/wiki...Flash+Settings
explore it and understand it to know better. But I would personally recommend stage 2 (also the most popular if I am not wrong.)
** BOT-ON **
You would just need a downpipe.
Charge pipe upgrade is not required for a stage 2 setup but given that the OEM one is plastic and might get brittle and crack as the miles pile on, for the peace of mind and given its relatively inexpensive given that you are getting work done on your car, might as well upgrade it now (prep for a stage 3 in the future)
For the downpipe I would recommend going for a good catted one. (AA catted downpipe)
Personally for me the smell is not worth it to live with and emission testing is not scary.
It still good for upto 800whp (fact check) so you are not missing out to a large extent.
Not required but highly recommended is installing a MHD flexfuel kit (TU Pump required at minimum for running upto E30-E40 mixture) that provides real time E content reading to your ECU and flashing a flexfuel map with it, allows your ECU to adjust power on the fly and makes the tune even safer and flexible.
For example if you flash with a base map of 91 Octane, your car would but safely optimizing for the max performance right from 91 Octane -> 93 -> E40 mixtures and you do not have to worry about the fuel quality.
E30 experience is superior to just running 93 Octane
** TRANSMISSION **
I have had the reliability concern for our transmission for the longest time and still am careful about it.
I think our zf8hp50 gearbox is still pretty safe for a stage 2 setup and even upto a good custom tuned stage 3 setup where the tuner has been careful with the torque output.
XHP transmission tune stage 2 on the contrary helps with a tuned car by increasing the clamping pressure and removing the torque limits that is needed with the increased engine output. You should be pretty safe there with a stage 2 XHP tune.
STEPS.
1. first get the MHD universal adapter (~$80) and install the MHD app for free and check if your car actually needs a FEMTO unlock (my guess is given its a 2020, you 99% chance do not need it) The adapter is super handy and allows you to work with bimmerlink, bimmercode and flashing MHD maps and flashing XHP map by yourself, also clearing out error codes. Following steps are assuming you have the Universal Wifi adapter.
2. XHP tune can flashed independently to an engine tune. your TCU is unlocked even if your ECU is locked. you can flash on a stock car, stage 1, stage 2 setup. Stock car also gets benefits but becomes mandatory with a tuned car due to reasons mentioned above.
once you have the adapter you can flash the XHP tune after purchasing the license online and by yourself. This, I did it myself as well. Look out for discount coupons before buying, might get lucky and save some money there.
3. Next up, buying the MHD super license. Once you buy it, without even buying installing an boltons and assuming you do not need a femto unlock you can flash Stage 1 tune by yourself and that should give you a massive increase in power compared to stock. By this point you have probably spent under $1100 dollars for a stage 1 tune and xhp tune and getting biggest power jump relative to a stock setup. You can run this setup for a while enjoy it or do it all at once an start with stage 2 (with flexfuel) directly, upto you.
EDIT: You would need a bench unlock which can be done for much cheaper.
4. for a stage 2 move, have the bolt-ons installed, flashing a stage 2 map from the MHD app is no different that flashing a stage 1 map.
** RELIABILITY **
There are many people who have put around 100k miles on a stage 2 setup and its pretty reliable. Make sure you are up on maintenance and you should be good. I personally have halved my oil change interval as I mostly run E30-E40. thats about it.
Enjoy.