I still have a 99 Series I Discovery, never had a large problem put 90k miles on it. Used it a little offroad, has a detroit on the rear TT on the front. Some of them go forever without a problem, others have one problem after another, quality control has never been great :0 Setup correctly they are very good offroaders, but they will never be a rockcrawler. Aftermarket parts tend to be on the high side of price. A lot of people put Toy axles on them for strength and cheaper to get parts for. The Buick engine they come with imho is trash. aluminum, leaky no matter what you do, underpowered for the size of the vehicle. The trick setup is a TDI or to add one but the cost is insane.
WWW.Discoweb.org has good technical help on Rovers and a lot of good links to parts and documents on repairing them. I really liked mine but the problem was it didnt help me as much as i needed at work so I upgraded to a truck. I also have a 95 Discovery I bought as parts, and a running 78 Series III right hand drive diesel.
Leo