Early MK3 calipers are the same as the MK2 ones. You need late spec ones. You can tell they are right because the cable mounts on the underside, not the top.
The MK4 calipers are the same as the MK3 ones, just alloy (no rusty). They have a banjo fitting, so need a hose conversion kit. If you're going from drums, you'll need the 4 hose, from disks you'll need the 6 hose IIRC.
For both of them you need the late spec handbrake cables, pre 88 cables won't fit right.
You can mount them the wrong way round (left on right / right on left) and retain the pre 88 cables, but the bleed nipple will be on the bottom - so you'd need to remove the calipers to bleed them.