First things first the diesel
is slow, but depending on what you're used to or your driving style this may not trouble you. I can't speak to reliability vs V8s with ownership authority but have a look through posts on here to get a feel for what people are saying. As you can see, no P38 is what you'd call a safe bet.
The other element to consider is kit. The V8s (especially the 4.6s) were the top of the range so came with more wood, leather and equipment than the entry level diesels. There are higher spec diesels (dHSE) but these aren't common. Again this may not bother you but is worth being aware of.
On your last point, an old P38 will be an expensive car to run if you use a mechanic for everything.
The newest P38s are nearly 12 years old and they are not built to Germanic quality standards, so things will need doing.
I'm not especially mechanical myself but am using P38 ownership as a means of teaching myself how to do some mechanical tasks. I quite enjoy tinkering these days, though I have the space to do it luckily. When I was 21 I didn't have a garage of a selection of tools, but I'm not sure of your circumstances.
Best of luck with your decision and have fun finding the car you want.