It all really depends on what your music production needs are. Most would advise you to get an audio interface that meets your basic needs plus a little more so that you have some room to grow. Fortunately, there are lots of options available.
If you plan to do electronic music, you'll definitely want something with Midi I/O. These days it's really easy to find an audio interface that has both Midi I/O and mic/instrument pres. Rather than just get a midi interface, a combo gives you the flexibility to be able to record vocals (or sound samples), use condenser microphones and plug in instruments like electric guitars as well as a keyboard midi controller. If you're set on sticking with Focusrite for your audio interface, their base model firewire option is the Saffire Pro 14 - which has 2 mic XLR combo inputs on the front panel with phantom power, plus 2 1/4" TRS inputs, 4 balanced outs and Midi I/O on the rear panel.
Some other notable audio interface manufacturers include Presonus, Avid, Tascam, Roland, M-Audio...and on the higher end: RME and MOTU.
This firewire expansion card has the TI chipset if you do decide to go with a firewire interface.
For beat design specifically, another option is Native Instruments' Maschine (or Maschine Mikro) which is a midi controller designed specifically for makers of electronic music. NI also has an audio interface that is receiving some attention - the Komplete Audio 6 - which might be worth a try. Native Instruments makes amazing VST's and I would give their hardware a shot if it is at all at the quality level of their software products. The Komplete Audio 6 is USB though, so keep that in mind.
What music production software do you plan on using?