Because the Death Guard can't take heavy weapons, our long range capabilities are HORRIBLE. As far as long ranged stuff goes, I usually stick with just a Predator with the tri-las config. or a drednought with a TL las. Defilers are good in certain situations, but typically are too unreliable to depend on.
With T5 and true grit, plague marines are kick butt in cc against most armies. I find that the best strategy is to load as many plague marine squads as you can into rhinos and advance toward the enemy 12 inches at a time. Once within range, pop out behind the rhino, and assault the following turn. Basic DG strategy there. If you keep distance between yourself and your opponent, you'll most likely get blown away by autocannon/lascannon/heavy bolter/(insert any gun here) fire.
Also, a lord or DP with a manreaper, d. str., d. mutation, and d. speed will tear through your opponents infantry squads at a time. I mean, 10 power attacks on the charge? Thankyou Grandfather Nurgle.
-bigPappa