When DVC lowered the minimum below 210, a disclaimer was added that "some aspects of the program may NOT be available at this level of membership". That was directed towards the exchange program, where some exchanges may not be available for a fewer number of points.
In actual practice, there are many opportunities for exchange for 180 points or less and MS does a great job of finding these for members. There are certainly exchanges which will require more points, but many can be found for fewer.
The disclaimer mentioned above certainly wasn't seen as a major obstacle and most members were never affected. It was made more as an advance warning to ward off later complaints, IMHO.
There is no minimum, as all exchanges are made on a case-by-case basis.
Enjoy!