Well recyled teenager Pete, it all depends on how you use the car and the extent to which you are dependant on it. I have just had a TIA and the 4 weeks that I was unable to drive have been an eye opener for me about how dependant my lifestyle is on my driving. From what I have seen, a detached retina is eminently repairable, and once you are recovered it should be no further problem. So dont change lifestyle in a panic.

If you are serious about going down to 1 car it has to be reliable with easy dealer access for servicing etc. That means mass manufacturer. Personally I would go for something like a 3 series convertible, maybe even an M3 if finances go that far. Maybe a C class convertible. Either way they will be moderate fun to drive , reasonably reliable and with a handy network of service spots.

They wont substitute for the interest / fun of a roadster.

Me? In your position ( and I'm not that far away with a hole in the macula of one eye and after the TIA) I would keep the Roadster which wont cost a lot to run, and get rid of the tank which is a reliability problem wating to happen