Ok, I think I concluded to a design I'm comfortable enough with testing. It requires threading the 35mm part of the R8 spindle, so I can use a set of 7207BEGAP angular contact bearings. They are universally matchable, so the inner races are factory ground for a light preload, so fitting a KM7 nut will secure and preload them. Top bearing will be a standard deep groove 6206. The lower gap of the pair will have a flange that will secure the outer races and house a 40x62x8 radial shaft seal, series CRW1 or CRW1A.

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Also, (assuming the rest of the design works) the more I look at it, the more I don't like the design of the lever operated drawbar. The benefit is that you can use a small and cheap pneumatic cylinder, but the end result will look a bit bulky and amateurish to me, although fully functional. If I can spare the expense I might change the design to a 100mm bore 10mm stroke air cylinder. I think I'll need about 6mm of travel for 3 pairs of washers to release the collet, but I'll test as soon as i get them.