Some feedback on my experience if it helps anybody else.

I have in my possession both ST170 calipers & Mondeo calipers. Mondeo definitely have larger piston size (and physical size). I measured and thought I'd remember - I didn't. I think Mondeo was 5-10mm wider. I could be wrong here but I'd think brake pad size & disc size is probably more important than piston size for a non-performance brake setup. I've gone with the ST caliper myself, mainly because it's in much better condition than the mondeo.

I fit 16" alloys in the end so didn't need to grind. So cannot comment on that (went for 195 tyres in the end)

Spacers for caliper. I used some steel rods with M12 hole and cut down to lengths of 12mm. This is too short as Micky said. 12mm length just about cleared the disc but I wouldn't want to drive continuously like that. I'll be trying 15mm once they arrive.

I didn't replace the brake hoses so running with the standard ones. Although they aren't in the best state so will probably replace with longer ones. They seem to be a bit stretched for my liking but so far functioning ok.

Brake fluid was well overdue a change too. It was not a good colour! Should really bleed the rear too.