Long time lurker, first time poster. I need some help with a new F15 or to keep my E70 and DIY long term.

Here is what I have currently:

2013 X5 50i
Loaded, every option
49k miles (bought with 28k)
CPO warranty until 9/2018
Extended maintenance plan until 9/2018
KBB value: $28k. I owe $28k. It has depreciated faster than I can pay it.

I have interest in learning to DIY this vehicle but am scared away by the costs and difficulty of the N63 engine. I am a novice DIY'er but have done extensive work on my E46 330XI.

Since the car is depreciating so quickly, I am floating the idea of trading it for CPO '15 F15 with the N55 engine. I know this is a very DIY capable engine since I can reuse some of the skills I learned on the E46.

Long term ownership an X5 is my overall goal vs my past BMW purchases where I dump them just before they are out of warranty.

I have also considered an F15 35d but was concerned on DIY difficulty.

I plan to garage this vehicle and daily drive my E46. Long term ownership, and ability to pull a trailer and good reliability (compared to other BMW options of course) are top of my list. Oh and comfort seats

Thoughts and advice?