If its just surface rust - then replacing isn't really necessary and a skim should suffice.
1 disk, not that expensive, 2 disks - still not that expensive, 2 disks, 2 callipers, a set of pads, brake fluid ..... it all adds up.
As long as you're not skimping on safety, there shouldn't be any real issue - save money where you can I say! Helps keep more MK2s on the road and more money in our wallets!