The 2.2Kw model is 80mm dia. It varies for the others.
As for Spindle mount options your looking at making your own really. You can get them from places like ebay US and various CNC retailers over there but shipping makes it expensive.
If your feeling adventurous then using an SMA50 linear bearing housing with the bearing case removed provides exactly an 80mm dia bore. This is a very sturdy mount for the spindle since you've got over half of it clamped. You do have modify it a little though by cutting a channel in the side for the clamping action. Pretty simple really if you've got the right tool for the job(band saw, radial arm saw, a jigsaw with a good metal blade or if you clamped and set up a jig then a regular hand router with 3mm dia straight bit would do a great job). I'm surprised some enterprising sort isn't knocking these out to be honest. There's certainly a gap in the UK for it.
The housing is £21+vat from Gary at Zapp.
SMA50UU Linear Bearing with housing
Attachment 7797 Attachment 7799