AcidChildren... lots of options mate. I think if you want the full value of adding percussion to the live rig you will go something you can play with sticks. The SPD6 is the smallest but mis-triggers a lot, so mine sits in the band storage shed with a heap of other stuff. The SPD20 is a good bit of kit though, pretty much the status quo of trigger units. From here you can add in extra percussion heads as it has trigger inputs so will act as a head unit and its reliable if a little stuffy with its midi out. All these units are really is a form of piezo element that turns the hits into electrical signals, along with circuits to convert that into sound/audio/midi. You can actually make your own piezo drum triggers really easily, and then get something to convert those signals like the Alesis D4, which would rack mount easily. You could also harvest parts from a cheap midi drum kit like the Legend range (or the Roland TD range, or the elusive but awesome Clavia DDrums). Really comes down to the form factor you want on stage and the ergonomics.
Basically lots of options including the Kat unit i linked above. In Segue we no longer bother with any of this, and even my midi drum kit has joined the acoustic one in storage. I DO do some live drum triggering on the MPC but thats kind of off the cuff improv over sequences. I would in your case suggest going a SPD20 and using Battery2 in your Ableton rig. How many laptops on stage though? I wouldnt put too much strain on it if it is your main sequencer too. Still, this lets you add more percussion (preferably the mesh head stuff from the Roland range) as you need and you can also have a midi unit like a BCR or any controller to control sends in Battery. We run one laptop as master Ableton and another in sync, and I have various bigger samples the MPC cant fit in Battery run from the MPC and separate channels/fx/sends in Ableton for it mapped to a BCR.
Sorry for the rough rambling reply, still have a cold and tired (lets call it "tiredness") from last nights gig.