Surely your shimano disc brakes are post mount already and use an IS to post mount adapter. They should just bolt directly up for a 160mm rotor or use a post mount + 20mm adapter for a 180mm disc.
I wouldn’t take the rebas, as above you could get a 2012/13 version for less than £300 if you shop around but out of the 2 I would go SIDs and save £150.
But I would also go with a carbon rigid fork for that. Will make the rough track much more fun tbh and be a lot better for the road.
edit: ahh no, you have old cable brakes. Looks like they are not post mount capable unless you buy a post to IS adapter and possibly a 180mm disc and keep the existing adapter. Or just buy a new post mount caliper for a few £ and swap it over. Easy to do with cable brakes.
Link: http://forums.mtbr.com/brake-time/post-mount-adaptor-help-739314.html