Hmm unfortunately i don't know much about the Korg ER-1, so i can't tell. Don't know in which songs Liam have used it either. He's getting stuff free from Korg so.... but maybe they're in some songs we haven't heard yet. Anyway i think you can get some nice stuff out of them (the factory demos usually are crap).
I think that the MC-505 has mostly common sounds, if you want that. But i prefer samplers for drums.
Personally I'm using a lot of drums in my songs, and a lot of different methods too. Sometimes I use the W-30 for drums (me? a Liam wannabe? nooo ;-). Say, if a loop sounds crisp, i sample it into the W-30 - thats my way to get a little bit of that ruff 12-bit sound :)
But most of the drums I use in a song is from the Akai sampler. Nowadays it's a S3000XL, I have used a S2000 before (still have it). They are very nice and quick to work with even though they have this tiny screen. I bought the S3000XL second hand and a Syquest EZ135 Zip-Drive followed, and i've noticed that Liam has one too.. hehe just a cool coincidence.
Create a program on the akai with a lot of cool and fresh drumhits organised. Then just play and jam over the keyboard, a good way to come up with some nice rhythms.
I also use s/w Fruity Loops for drums, mostly when i want some nice shuffle to my beats ('caus i think it's hard to quantize nice shuffle/triplets via midi, but thats just me..).
Well Ekko said that Liam uses the old E-mu SP-1200 for bassdrums on "Fat Of The Land".. so there you got it, just buy one ;) There's only one disadvantage with them, they aren't cheap at all.. Maybe an AKAI MPC-60/MPC-2000/MPC-3000 are the best choice (?), anyway they aren't as expensive as the Sp1200. Many great prducers are using these Akai's too. Like BT, DJ Shadow, Cari Lekebusch, Crystal Method, Chemical Brothers etc.
But you said you can't afford it, hmm so just.. wait and see what Ekko answers :)