I am looking into getting a 2000 or 2001 Discovery. When I do I would like to up grade the suspension with a OME lift, and I want to get wheels and tires. I want a 15" black steel rim with, maybe, a 31-10.50-15 tire What should the rim specifications be. Such as offset, and bolt pattern size and a good brand. Also is this a good tire size?

 

There are few companys making whells for Rovers, so you won't have a big selection.
Once you buy the truck and decide on how big a lift, what you want to do with the truck and how much drive line work you are prepared to do, then start looking at tire sizes. You will need to look at axle changes, possibly drive line up grades as well as diff up grades.
I would rather see you buy the truck first, spend some time off road getting used to it, the see how much money you have to spend.
Mike