Since there are so many audio interfaces available with vastly different input/output capacity (number of channels) and vastly differing quality levels, and a very wide range of prices, it's important to carefully decide on your current and possibly future needs and the quality level you can afford. If you're willing to pre-mix your drum tracks down to (2) channels, a mixer can be used ahead of a pair of interface line inputs, or you can purchase a mixer with a Firewire interface for all channels. There are plenty of larger interfaces with 24, 28 or 32 channels (expanded via ADAT inputs and/or external mic pres) such as the MOTU 828MK3 or the RME Fireface 800 that do include built-in MIDI. Either of those can handle a full drum kit, guitar and MIDI keyboard. Doing a little basic research now can save you time and money in the future.
The Utilities -> Audio-MIDI Setup Control Panel lets you select each one independently. An interface with both audio I/O and MIDI will show up in each section of the Audio-MIDI Control Setup panel (after installing an appropriate MIDI driver if needed). In fact, on a Mac, it's actually possible to select several different audio interfaces simultaneously (a process called forming an aggregate device) to increase the number of parallel recording tracks.
If you use Logic, Cubase or ProTools as your DAW, you can just record enable both your audio and MIDI tracks simultaneously to record your MIDI keyboard while you're recording voice or drums/guitar (or anything else). Once installed and originally selected, there is no need to choose between audio and MIDI. Although different in the way they operate, audio and MIDI tracks can co-exist side-by-side in the same project. Just enable the appropriate tracks and record each or both types of inputs as you please.
Hope this helps.