Most prevalent issues include, but are not limited to, thermostat failure, VANOS solenoid failure,hydraulic lifter failure and water pump failure. These jobs can get very expensive very quickly if addressed late. It is recommended to do these jobs as preventative maintenance before these compound issues over time. Ideally, you’d want parts with warranty, so have a look at FCP Euro with their lifetime warranty.

Other issues primarly revolve around gasket leaks. No real fix to these. Just got to stay on top of them before they turn small issues into big problems.

If by head unit, you mean the actual iDrive centre console screen, that won’t make a difference at all, except for it being a lot wider than stock.

Realistically, if you fit a CarPlay unit to the car, the unit would have to be “married” to the car through coding. This could be achieved with something like Bimmercode, but more intensive software may be needed like ISTA. Once it’s married to the car, you might have to do some further coding to re-enable some things like navigation, parking cameras and radio antennas. Other modules might need to get to know the new module again, so that means more coding.

It’s not a hardware issue for the most part so much as it is a software issue. In my experience, improperly coded devices can lead to damage, or even potentially tripping component protection.

It may be easier to get something like a BimmerLink MMI retrofit kit. This kit is plug and play for the most part and doesn’t require a lot, if any coding. Plus, it uses the default Apple CarPlay system that BMW uses, so it still maintains the OEM aesthetic.

Out of all the engines, bar the diesels, the 630 is the pick of the lot. The gearbox is kinda the Achilles heel of the whole deal though, though this is true for the entire E6X range. These cars are generally reliable for the most part so long as maintenance is half decent, and all major issues with the N52 are addressed.

It’s been a good while since I last worked on a E63, but IIRC the transmission can be remapped relatively easily to make it more easier to drive on a daily basis. In some cases, the flash improves fuel economy, depending on your commute.

That said, the E63 has other issues as do other BMW models from this era. Most are electrical issues that can lead to more severe things down the line, like a old battery causing the car to go into limp mode due to low voltage, or the steering rack locking up or becoming heavier than normal due to the pump dying. Other issues include the headlight computers and the ECU. Most of the time, this is due to faulty wiring running to the ECU.

There are a few other issues, but I can’t think of them at the moment for the life of me. These issues are to be expected, as this is an older BMW after all.