Maybe go for the older M3W and spend the £5k on a second car?
This sounds like a damn good compromise. An earlier car with the comfort kit and Centa drive unit would make lots of sense.
I use my M3W all year round but don't have to rely on it for transport - I have an old anonymous Honda grey box for that.
Driving the M3W in proper cold weather isn't a great problem but the dressing up bit takes time which can be a bit of a pain.
As an aside, when my brother was newly qualified he ran a Lotus 7 for a couple of years which made for some interesting journeys but it did have a hood of sorts and a heater for burning his feet. He did on one occasion have to drive in it from one end of the large hospital he was working to the other with a colleague, in their white coats with the colleague holding the green flashing light up at arms length to get through the traffic. Such things are probably frowned upon now.
