42k is not high mileage. I picked up my P85D with 47k on it last month. Battery degradation is about 1% so far.
The + will have stiffer suspension consisting of upgraded air dampers, thicker sway bars, stiffer bushings, and staggered 21" wheels. Many like it as it sharpens the steering and cuts out the floatiness of the normal air suspension. Some think it's too harsh. I have the + suspension on my P85D and I like it a lot, but my other sporty car is a 1st gen Miata with a lot of suspension modifications making it pretty darn stiff. I also have 19" wheels and Primacy MXM4s which contribute to a nicer ride. You can always swap wheels and tires, but swapping the suspension is a significant undertaking as I believe the motor(s) need to be dropped to access the swaybars and bushings, not to mention acquiring the parts.
That all said, I don't think the older car is worth the $12k premium.