For Mahler, if you strongly prefer instrumental music, I agree with those who recommend the first and fourth symphonies. They are relatively short, as Mahler goes. I wouldn't touch the fifth at this stage, although its adagietto is one of Mahler's most beautiful movements. If you like vocal music, I'd look for his orchestral song cycles, Songs of a Wayfarer (echoed in the first symphony), Ruckert Lieder and Kindertotenlieder. Then Das Lied von der Erde (Song of the Earth.) Jim Tobin