I have a stock Disco03.
What kind of slopes can it handle.
45 degree?

 

I'd consult your owners manual. If I remember right they give you max. angles but not sure with that year.

 

What sort of 4 wheeling experience do you have?
If you are talking about a bone stock Disco with 18" tires you will need to make some changes before you can tackle that sort of angle. Besides you don't have lockes and your diffs won't put that much trction to the ground.
Mike