GLS and CD are model designations which are reflective of the trim and equipment (or toys). GLS is generally the lowest model although you may see a reflection which I believe is lower still. CD is slightly higher than GLS with wood trims, electric front windows better stereo and air con, CDX better still with climate control, electric windows all round wood trim on doors and centre dash etc. Top of the range is the Elite which has all the CDX has and more, like fully upgraded stereo (Bose), auto dimming mirror, memory seats, leather trim (standard on petrol 3.0 V6) body coloured sill moldings etc.
As to whether there are a lot of diesel 2.5 models around for the year you are looking at, I am not sure I am afraid. I think that high spec cars were generally petrol models - diesel elites seem to be quite rare.
I am sort of on the look out for an Elite at present, but the 2.5 V6. Good luck with your search
Hope this helps