If you got yourself an Akai MPK61, you'd have a 61 key keyboard with 16 pads and a bunch of controllers. Korg also do a 61 key version of the Korg Kontrol keyboards... again, they have 16 pads.

The difference between the MPD24 and the MPD26 is (as far as I can tell):-

MPD24 has 8 rotary encoders - MPD26 has 6
MPD24 does not have 'Repeat' function - MPD26 has repeat function (this allows you to tap in a tempo and then press the repeat button... you can pick a repeat speed and then when you hold down a pad, it plays it repeatedly at the given speed taking into account the pressure you're applying)

That's about the only differences I can see... so, if you don't want that functionality, go for the MPD24 and get 2 extra rotary encoders.

As I said, the only reason I didn't go for something like the Akai MPK61 or the Korg Kontrol 49 is the fact I have a MIDI keyboard already in the form of my Alpha Juno 2. So rather than pay £400, I chose to pay £130 for the MPD26 and just get what I needed.

As for Audio/MIDI interface... I obviously have a bunch of MIDI gear so I bought the 8 way MIDI interface (for full use of all my MIDI gear I need 42 MIDI output channels + 4 MIDI input channels). If you don't need lots of MIDI, you could consider something like the MOTU UltraLite mk3 Hybrid. This has a bunch of audio in's and out's and a single MIDI interface (16 channels). Or... if you fancy spending some cash, the 828 mk 3 firewire interface. That seems really nice. I can't comment too much about pro audio interfaces as I don't have one, but there are loads of options. There are some good internal cards that would do what you want, but for things like this, I'm a firm believer in buying the best you can afford.

Keyboard choice... it's a personal thing... find a local dealer with a store, go down and try them out. The one overriding thing I'm finding is that even though these things are costing the same as they did say 10-20 years ago... the build quality of the units you buy today just doesn't seem as good.... my Alpha has a metal chasis... most of the stuff you buy today has never seen metal. But get yourself to a local dealer and try a few out.

If I had the cash, I'd probably buy an Akai MPK88 which is 88 keys with a piano weighted action. You just can't beat playing a piano.

Setup questions... well, if the devices have USB interfaces, I'd be inclined to use them. MIDI operates at something like 38400 baud... compare that to the crazy speeds of USB 2. Don't dasiy chain too much. Thats why I bought a fat MIDI interface. The more daisy chaining you do, the more delay you add to the signals and it can become obvious. The same is true however of USB. You're not supposed to have hub linked to hub linked to hub etc. If you've not got enough ports, get yourself two USB hubs... one for the music gear and one for everything else. Things like audio interfaces... I would connect them directly to the USB on the motherboard rather than through a hub as you want to do everything you can to reduce latency.

Any more questions/clarity... post away :-)