Hi
@Sherminator, I've had my Z4M E86 since the start of the year and I did the same investigations you have no doubt started. I wanted to add GPS and DAB+ to the car without having to go through the phone.
[Edit: Apologies, after writing I realised this was a bit of a brain dump]
As
@feket663 mentions the first important question is exactly what stereo system you have in the car because the fancier it is the more fiddly it is to change. You can find this out by
going to an excellent site owned by forum member @tonyz4c which can detail all the options on any given car. If you put your vin number in and click on the matching line it will show a window with all your options. (You can narrow it down by selecting some of the drop downs, e.g. i always select the area on the map then the zone, model and colour). I have the "S662A Radio BMW Business CD" with no extra speaker options which is the simplest setup. There are some good postings in the forum about working with the various stereo setups.
When it comes to what options are available there are several that I considered.
Next question is proprietory head unit (Alpine, Pioneer etc) or Android. Proprietory means company specific software but everything should work out of the box, Android head unit means there might be some configuring needed but once working it will behave like any other android tablet and you can install any app from the google play store and it should work.
So options, first if you have and want to make use of a flip-up screen area of the dashboard:
1. If your car has the flip-screen in the dashboard then you could replace this with an android head unit. Before I bought my car I was looking for one with the flip-up screen as this was my plan. I started talking with the people at hifimax.net about options. Their tech support was excellent and responded to my endless email questions very promptly and politely. For our E85/E86 models it would be
one of these Android head units. These are fixed screens so would no longer flip up and down. I cant speak to the final quality as I ended up getting a car without the original flip-up screen and so going a different route.
2. If your car does not have the flip-up screen it is still possible to go with the hifimax option but you would have to cut a whole in the dash. They provide templates and instructions for doing so.
3. It is technically possible to install a homemade solution.
@danmiddle2 wrote up
an excellent how to guide explaining how he replaced the standard flip-up screen with his own custom build using a standard android tablet. This clever approach has the added benefit of the new screen still using the flip-up mechanism.
If you dont have the flip-up screen or dont want to use that part (for example some people have removed the flip-up screen and just turned that area into a little storage area) then there are a couple of other options using the 1 DIN slot that originally houses the stereo head unit.
4. 1 DIN stereo with fixed screen. There are head units out there which have a main body with a 1 DIN sizing and a fixed flat screen on the outside
such as this. A quick search of the web will find
web pages like this showing a few examples. Some have quite small screens because of the single DIN form factor, some have large screens that stick forward. On some the screens are removable if you wanted to hide the screen in the glovebox when you leave the car. Some of the screens are nice and large so easy to use but some might consider a downside that the air blowers cannot blow into your face as the screen would be in the way (there would still be air flow just not directly at you)
5. Lastly (and this is the option I went for) there are 1 DIN head units which have a slide-out screen. This means the screen can retract back into the head unit when the engine is off. The screens aren't as large as those on the fixed screen units as they obviously have to fit inside the 1 DIN body but they are usually 7". There are a few proprietary models such as
the pioneer or there are android variants (there are a whole bunch of android flip-out head units to choose from).
After all this investigation I ended up choosing to go the Android flip-out screen route. To give you some idea of what was involved, in my case I went for an
Erisin unit. Erisin seemed to have a bunch of variants and this was just the tartiest one

I also bought
their DAB dongle (most android head units need a separate DAB dongle that connects via USB, there are many makes of DAB dongle and they should all work).
I have installed the head unit and most features like GPS worked straight away. I am still working on connecting up the antenna for the DAB dongle so dont have DAB yet. However it supports both Apple Carplay and Android Auto. I have already connected it to the house wifi and downloaded a new GPS app (Magic Earth, which I prefer over Google Maps) and the Brave web browser via the Play store and searching the web works too. This unit also came with a connection for a mobile phone SIM so if I wanted I could install a SIM card and the car would have full mobile signal whenever there was phone coverage without needing my mobile. It has connections in the back for a reversing camera which is something I plan to install in the future. When I get everything working how I like it I will probably make a posting in the how to section with photos in case it helps anyone in the future.
I hope some of this waffle was of use to you.