Oh ok, cause what I really want to do is the audio transfer, I'm gonna do all if my mixes on my computer anyway.
Thats the most flexible way for future alterations to your files. Plus if you dont do it this way, your limited to only what your workstation is capable of which can obviously suck - so your better off tracking to the computer for further audio manipulation.
So if I wanna do audio transfers I need a S/PDIF cable and two midi cables.
Essentially this would be the most cost efficient / reduced way of still being able to achieve the same results versus buying a group of instrument cables.
(Note that the (2) MIDI cables are required for this task regardless in order to sync to your DAW via either MTC or MIDI CLOCK.)
IMO - you actually recieve better results via S/PDIF anyways when tracking versus the analog outs...But thats also highly dependent upon your preamp, your interface, your converters and so on. So theoretically, you can actually get great results both ways but standard gear (interfaces) that usually run under the 500 dollar mark maybe with the exception of a few like the new Apogee Duet are going to have so-so converters and so-so preamps, hence your better off running S/PDIF anyways unless you working with more sophisticated equipment.
Now is that just for the workstation or both workstation and drum machine??
This totally depends on the devices available I/O. So for example if you want to run S/PDIF out and sync via MIDI - your going to need to make sure that your workstation/sampler is equipped with these connections.
You can easily check this by just going to Sweetwater, Zzounds, or the Manufacturers site and viewing the available manual for the device in PDF format as well as taking a look at the high res photos that are usually available.
The new MPC models from the 1000 up are all equipped with these I/O connections. Same goes for something like the Fantom. But always do your homework and pay the manufacturers site a visit and read the manual and look at the photos.
How big should my interface be, as in how many in and out spots should I have on it or S/PDIF spots?
This is more a question that you need to ask yourself first...For example, if you plan to track via S/PDIF for a long time and dont have aspirations of being able to track other parts of your song simultaneously in groups then you dont have to go real big in regards to I/O...You just need to make sure that your interface is equipped with at least S/PDIF obviously and MIDI, and then whatever additional I/O may be needed for other gear or tasks in your studio.
The general rule hear is your better off getting something with more I/O typically so that if later on your recording habits change - your gear can still accommodate that.
On the other hand, if your ballin on a budget like many of us are (including myself), you might consider looking into something thats not as flexible at least for the time being.
If your still looking into a interface and your in the market, i would consider the Apogee Duet. But theres an unfortunate catch here: the Duet does not have S/PDIF, but it does have (2) line ins. In that case you would purchase (2) MIDI cables and (2) TS-TS cables instead, but i guarantee that this interfaces converters will be the best in this price range, plus your getting better on board mic pres versus the competition in the same price point. Just a thought.