Hi Bluejay welcome to the site.
all these views and no one bites?
well lets start it rolling
The Original Bmw wheels have the hubs fitted so this is the easier option.
If you have the original hubs ,you can get wheels built using other rims such as excel etc.but of course more euro's.
You can put a 21" on your standard F650,just need to raise the forkbrace with spacers.A better choice than the 19"not sure about that one,
prices variable,the wheels are not easily found yet.
you will additional info on the F650UK site and chaingang sites.