I would go with BMW as an investment , M34 m5
and E38 750
The e38 750i,s are already steadily climbing in value.
I may well buy an e38 in the near future as a "weekend" type car. 3 - 4 thousand miles per year being the likely mileage.Its much more likely to be the 740i than a 750 though. Running costs of a 5.4 V12 are beyond horrendous.
An early M3 may be a sensible investment classic though.
Having said that, Im leaning towards guitars now, as they take up a lot less space, and need very little maintenance etc.
On 3 to 4k miles a year, I doubt you'd notice much difference between a 740 and 750. All the bits inside and underneath are the same; only the engine really differs. And they're both big engines that don't need to be worked hard so I the economy won't be
much different. Both are expensive to own, but you might find the 750 costs the same or even a bit less to buy too. Although I can't see much point in a 7-series, a manual 540 could briefly be fun in a very dull sort of way.
You've definitely missed the boat for M3s; early ones were always rare and expensive, E36s weren't rare and consequently suffered from the high
I can just afford to buy it/can't drive it attrition rate like Cosworth Sierras. Later ones are much less special and are much less likely to be 'collectible.'