Cost of fuel isn't a concern here - I've run V8 muscle cars before and seeing anything over 20mpg was very.. well, it just didn't happen.
I'm all for a hobby car.. but when I get in and turn the key I want to be able to drive it. If some of the electrics have suddenly stopped working then fair enough.. but it needs to get me from A to B. I do have other cars.. but if I wanted something that was going to break down constantly.. I'd be looking at an Alfa.
Maybe a P38 isnt for you... I think generally people would agree they will get you there to a fashion.
Overheating is a big killer of them all with regards to "getting you there" Air suspension can fail but its designed so it can be driven on the bump stops at up to 35mph however I wouldnt recommend it. Compressor failure subjected me to some bumpstop action (although without messages) and changed the character of the car to such a degree we were not on speaking terms until the new seal kit arrived.
As for 20mpg if you are looking for that you will need a diesel, you'll get that at 65-70 out of a V8 on a gentle run down the motorway but bet on around 14mpg average and if you manage 16 give your self a pat on the back.
I would expect a diesel should touch high 20's maybe even knock on the door of 30mpg if you are careful and probably around 20 knocking about. Factor in the extra cost of fuel and the cost per mile probably isn't much difference
Best thing for you is to drive both, everyone has a preferred choice some people even like the noise of the diesel! (To be fair, its quite a good noise for a diesel!) and then pick the one that suits you. And then find the one in the best condition (youngest and lowest mileage means nothing on these and nor does having loads of parts replaced... a bit list of "heres the bits I've changed" screams I'm looking for a fault and I cant find it so now I'm selling)