Hi Scott,
as Douglas says, a good 300tdi is probably the best bet but be very careful what you buy. Rust is the big enemy and Disco's of this age are very unlikely to be completely free of it. Check the chassis, lift the carpet and the rubber mat in the rear load area and check the floor, they are notorious for rusting and expensive to replace. Also have a good look around the sills and under the 'A' posts under the front doors, open both rear side doors and check for rust on the curve of the wheel arch. And don't forget to look under the bonnet and check the tops of the wheel arches.Don't buy any Series one or two Disco without looking at that rear floor area.
For your budget, you should be able to get a nice one, just be careful.
Good hunting!