If a D90 with that type of mileage is regularly maintained, it will still have some issues based on age alone. But if all the service documentation is available, you will know what upgrades/replacements you will need to do in order to keep the rig running well.
My rig has 111K miles on it and has had relatively good maintenance records from previous owner so I feel relatively confident that it will keep on going as long as I keep up with the maintenance.
When I got her, I immediately did a 90K maintenance regimen, replaced the dangerous oil cooler lines that could blow up a defender, and also got the recommendations for future work to be done. Of course all of this has a price tag that comes along with any higher mileage vehicle of any kind. If you are mechanically inclined, you can probably save yourself alot of cash.
My wife and I used to have a DII and traded her in for a LR3 after the warranty was up. To be honest, it would of been a real gamble maintenance wise to keep her. It was in the shop alot more than I thought it would (under warranty).
On a side note, the LR3 has been better than the Disco II we owned, but I can say we will probably be doing the same and trade her in after it's warranty expires. On a down note, LR3s are super expensive to mod and probably not worth doing unless you really don't mind dropping alot of coin.
As to your question regarding modding a Discovery to be (almost) as capable as a Defender...well there is a lot of great aftermarket support for Discovery I and IIs. and you can check out discoweb or if you are into expedition travel - check out the Land Rover section of expeditionportal. Lots of nice reference and member build ups there for Discoveries, Defenders, Series and more.
Good luck! I found this site to be extremely useful using the search tool and the members here are very knowledgeable