Series axles are fine and will cope offroad if you still have standard engine. If you have a V8 or a 200/300tdi under the bonnet you'll be going through diffs (even rangie ones) and halfshafts like hot dinners.
If you fit disco ones like I did you get the benefit of discs all round but defender or rangie will do the same job. Also the coilspring axles are I think about 3" wider each side so if you've got wider wheels you will need defender arches,but you can keep the original look by fitting swb rims as they just fit inside the series arches/wings nicely with the wider axles