Well, if you're NOT intentionally going for a Khorne army....
For most armies, Horrors are surprisingly effective. They may only be BS 3, but with all those shots and with the AP 4, most armies will be pretty devastated by a unit of 5-10 of them. Unfortunately, there's only one type of army they suck against.... Space Marines. So, in your particular case, you have the right idea.
Sadly, against MEQs, Bloodletters really are your best best for dealing damage, despite how squishy they are. However, they're really only going to be able to handle it if you have them in large units, at least 10 (or 8 if I'm feeling fluffy. Blood for the Blood God!) Plaguebearers, on the other hand, make up for not being able to break through their armor by being both cheaper and one of the best objective-holders in the game. If you want to give your Plaguebearers another chance, here's one piece of advice for them--since they're so hard to kill, don't worry as much about bringing them in in cover. Just drop them either right on top of the objective you want them to hold, or drop them in front of whatever you actually want them fighting. Still, if you want them for killing power rather than objective holding, Bloodletters are still the way to go.
You might also want to consider Daemonettes. While the Bloodletters will do more reliable damage, Daemonettes are faster, have higher Initiative, and their Rending makes them almost as good at killing MEQs. Heck, I've even killed a couple Dreadnaughts with them.
You have the right idea with the Bloodcrushers, although if you're dealing with Land Raiders, I'd only take one big unit of them. You're right in that LR spam is extremely difficult for Daemons. Unless you can get a Monstrous Creature or Soul Grinder close enough to smash them, that'll definitely give you issues.
There is one fantastic way to deal with Land Raiders that's often overlooked though--Flamers. Breath of Chaos automatically glances on a 4+. Now for transports and such that sounds pretty crappy, but an automatic glancing hit on AV 14 on the first turn can be pretty sexy. Even if you're not lucky enough to get 2 Immobilized results to Wreck them, you can at least shake them up a bit to limit their shooting. So, if the average SM list you face has 2 Land Raiders, 2 units of 3-5 Flamers to drop in front of them and glance them into submission while your MCs get ready to finish them off can be a HUGE help.
For that matter, have you considered Unclean Ones over Bloodthirsters? Even with all their upgrades they're cheaper than a bare-bones Bloodthirster. They're slower and don't quite do as much in CC, but they're much harder to kill, have that same Breath of Chaos attack I was talking about with the Flamers, and it's less tragic on your army's point total if it gets killed.
As long as you don't invest too heavily in them, Screamers can also deal with tanks pretty well. Jetbikes with melta bombs. 2 units at minimum size should get the job done if you don't want to take Flamers, as long as you keep them safely behind cover when they come in.
Also, don't underestimate the Soul Grinder. It's pretty much amazing. Most people only go with Phlegm, but I find Tongue is also nice, since it's the only other somewhat effective way to dealing with AV14, and you might get lucky as you march him up to smash the thing.
