Our LBS sells Both
Does the 520 disc have posts for v-brakes or cantis
No, Its Either Or.
My Koga Miyata WTR Has Both , and so Does My Bike Friday (but as a Built-to-Order bike that is a Buyers choice.
But you May be able to fit a Bolt on Side Pull on the fork Crown and rear stay If you really are a Belt-and Braces Worrier ..
And, If the Brakes are Cable discs You can always Pack a spare Caliper in your Pannier-Luggage . and a spare set of pads take up just a tiny Space .
How fast do rims wear with the rim-brakes?
There your service care the way you use them and Weather makes a difference
I got a decade of occasional Really Long European tours in , during the summer, and when I resold my Custom Built wheels they were still in Super shape
Filthy Rims Old Pads and dragging the rear brake a lot the wear will be shorter ..
(Moving to Portland, having my stuff shipped up once I get there)
PDXers ride in all weather on all sorts of Bikes .. wet weather the discs will stop faster .
The $110 is pretty cheap one caliper alone at retail costs More than that . (OEM factory costs are lower , because they get truckloads at one time.)
Heck once You're there have the rim brake posts added to the Disc Frame
(lots of Frame Builders in Town) and get a Paint/Powder redo in any color you want...