I guess the SLK 350 is an auto...

You'll have to drive them, the only way to find out. I certainly wouldn't be buying the BMW. Of the other two it would depend on what I wanted from a fun car. That's me done.

Nobody put the horse in the middle of the carriage!

At the same time, nobody ever took the engine out of the car, placed it 8' further down the road, gave it a very nervous disposition and attatched it to legs with which to pull the car...............!

As has been said, these are very different cars. But for me, the first to go would be the BMW. Not that it is a bad car, but in this company, it is just a bit 'meh'!.

The Mustang would be next. Hugely enjoyable initially, but i suspect i'd tire of its size, thirst, (relatively) poor quality, and brashness much sooner than i'd tire of the Boxster's dynamics......

....The Boxster (with a 6cyl engine) is a car i feel i would be most happy with for a long term ownership (relationship?). The engineering integrity, the quality, the sublime road manners. In addition, between front and rear 'boots', you have 275 litres of luggage space, so not at all impractical either. And the fact that you have a highly rated specialist nearby is surely a sign?!. One last thing, as the current Boxster has moved away from the 6 cyl engine, good examples will always be in demand, so i suspect the Boxster will retain more of its value than either of the others.

BTW, i wouldn't worry unduly about whether or not you'd 'fit' in the Boxster, German cars tend to be very accomodating for larger drivers. What may be an issue is that the Boxster is a very low car (10cm lower than the Mustang, itself not a very tall car), so the combination of your height and 58 years may mean some difficulty in getting out?!.

Nobody put the horse in the middle of the carriage! Nor did they put it transverse upfront, but I digress...

Does that mean you don't like rear wheel drive? A horse is at the front of a carriage, just like front wheel drive ;)

It really should come down a test drive and then the other subjective issues of looks image etc. As a Mustang owner I agree with Metroplis about perceived quality of German cars, a friend of mine has had numerous niggles with his Boxter in the same time that I’ve had none with my Mustang. Specialist or not the Porsche or BMW will cost a lot more to service than a Ford, even the Ford performance dealers charge standard rates. Also bear in mind that Ford expect Mustangs to be modded, this means that the stock engine and drive train is over engineered - you can safely increase power to c650bhp before needing to change anything major. Whichever car you chose, enjoy.

Never driven either a Mustang or BMW 435i but have had a Boxster 987 for last 10 years. It is an absolute superb car to drive. If you will be a low mileage user then the service interval is 20k or 2 years, not sure what it is on BMW or Ford but if annually you will save on servicing costs, although servicing is not cheap. Allow £1k for major service every 4 years, with minor service at £600 I have always had service done by Porsche main agent, could find independent specialist would shave quite a bit from this As mentioned above post 2016 cars were 4 cylinder turbo, model 718. Did test drive both the 718 and 718 S in 2017 but was not convinced enough to swap in the 987. Post 2016 cars don't have the 6 cylinder sound although Porsche engineers have tried to recreate this !

New Boxster GTS from this year I think has gone back to 6 cylinders !

I would thoroughly recommend the Boxster, fun car to drive, practical with 2 reasonable sized boots both 130 litres. Superb handling with mid engine and has been reliable in my ownership.

Insist on a full service history from either Porsche agent or trusted specialist, bodywork should be excellent as Porsches polish up well if looked after. There are a lot of dogs out there with big bills waiting to bite you, but if you get a good one that has been looked after you will be delighted.