I am trying to find correct discs and pads for my 1993 Discovery Tdi.
There are many options from same manufacturers, but also some solid vs ventilated?

Are solid and ventilated disc thickness's both the same? Are the corresponding discs interchangeable (ie same thickness)?

I currently have solid discs (stopping is great) and disc thickness is 13.5mm (above the min 13mm for these discs), however even with 6.5mm material on each pad the piston's seam out a bit?

Also, is it normal that the discs have a bevel (hence un machined) near the outer edge (refer photo)? It seams odd as the pad extends to the outer edge of the disc??

 

Im not the expert on this but as I understand vented rotors are not the same width as non vented and require a different caliper.

My 98 disco's rotors are shinny all the way to the outer edge

 

Maybe get new discs. Machined surface should go nearly to the outer edge, as mentioned above by Whiskey. A small lip, but not like your photo, pad contact area should all be machined.

Mud and gravel can get stuck in vented discs if that's where you drive. If stopping is fine, then brake work is not urgent, so take your time to plan the work. That outer rough surface is giving more grip, but will wear the pads quicker. Maybe the last set of pads did not fit well or were too small to get that unusual wear pattern. Looks like it lived in a salty area, check the chassis and underside well for rust and rot.